Libraries: Move to Userland/Libraries/

2025-07-27 10:47:35 +00:00 · 2021-01-12 12:17:30 +01:00 · 2021-01-12 12:17:30 +01:00 · 13d7c09125
commit 13d7c09125
parent dc28c07fa5
1857 changed files with 266 additions and 274 deletions
--- a/Userland/Libraries/LibJS/Tests/builtins/Array/Array.from.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Array/Array.from.js
@ -0,0 +1,69 @@
+test("length is 1", () => {
+    expect(Array.from).toHaveLength(1);
+});
+
+describe("normal behavior", () => {
+    test("empty array", () => {
+        var a = Array.from([]);
+        expect(a instanceof Array).toBeTrue();
+        expect(a).toHaveLength(0);
+    });
+
+    test("empty string", () => {
+        var a = Array.from("");
+        expect(a instanceof Array).toBeTrue();
+        expect(a).toHaveLength(0);
+    });
+
+    test("non-empty array", () => {
+        var a = Array.from([5, 8, 1]);
+        expect(a instanceof Array).toBeTrue();
+        expect(a).toHaveLength(3);
+        expect(a[0]).toBe(5);
+        expect(a[1]).toBe(8);
+        expect(a[2]).toBe(1);
+    });
+
+    test("non-empty string", () => {
+        var a = Array.from("what");
+        expect(a instanceof Array).toBeTrue();
+        expect(a).toHaveLength(4);
+        expect(a[0]).toBe("w");
+        expect(a[1]).toBe("h");
+        expect(a[2]).toBe("a");
+        expect(a[3]).toBe("t");
+    });
+
+    test("shallow array copy", () => {
+        var a = [1, 2, 3];
+        var b = Array.from([a]);
+        expect(b instanceof Array).toBeTrue();
+        expect(b).toHaveLength(1);
+        b[0][0] = 4;
+        expect(a[0]).toBe(4);
+    });
+
+    test("from iterator", () => {
+        function rangeIterator(begin, end) {
+            return {
+                [Symbol.iterator]() {
+                    let value = begin - 1;
+                    return {
+                        next() {
+                            if (value < end) {
+                                value += 1;
+                            }
+                            return { value: value, done: value >= end };
+                        },
+                    };
+                },
+            };
+        }
+
+        var a = Array.from(rangeIterator(8, 10));
+        expect(a instanceof Array).toBeTrue();
+        expect(a).toHaveLength(2);
+        expect(a[0]).toBe(8);
+        expect(a[1]).toBe(9);
+    });
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Array/Array.isArray.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Array/Array.isArray.js
@ -0,0 +1,26 @@
+test("length is 1", () => {
+    expect(Array.isArray).toHaveLength(1);
+});
+
+test("arguments that evaluate to false", () => {
+    expect(Array.isArray()).toBeFalse();
+    expect(Array.isArray("1")).toBeFalse();
+    expect(Array.isArray("foo")).toBeFalse();
+    expect(Array.isArray(1)).toBeFalse();
+    expect(Array.isArray(1, 2, 3)).toBeFalse();
+    expect(Array.isArray(undefined)).toBeFalse();
+    expect(Array.isArray(null)).toBeFalse();
+    expect(Array.isArray(Infinity)).toBeFalse();
+    expect(Array.isArray({})).toBeFalse();
+});
+
+test("arguments that evaluate to true", () => {
+    expect(Array.isArray([])).toBeTrue();
+    expect(Array.isArray([1])).toBeTrue();
+    expect(Array.isArray([1, 2, 3])).toBeTrue();
+    expect(Array.isArray(new Array())).toBeTrue();
+    expect(Array.isArray(new Array(10))).toBeTrue();
+    expect(Array.isArray(new Array("a", "b", "c"))).toBeTrue();
+    // FIXME: Array.prototype is supposed to be an array!
+    // expect(Array.isArray(Array.prototype)).toBeTrue();
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Array/Array.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Array/Array.js
@ -0,0 +1,53 @@
+test("constructor properties", () => {
+    expect(Array).toHaveLength(1);
+    expect(Array.name).toBe("Array");
+    expect(Array.prototype.length).toBe(0);
+});
+
+describe("errors", () => {
+    test("invalid array length", () => {
+        [-1, -100, -0.1, 0.1, 1.23, Infinity, -Infinity, NaN].forEach(value => {
+            expect(() => {
+                new Array(value);
+            }).toThrowWithMessage(RangeError, "Invalid array length");
+        });
+    });
+});
+
+describe("normal behavior", () => {
+    test("typeof", () => {
+        expect(typeof Array()).toBe("object");
+        expect(typeof new Array()).toBe("object");
+    });
+
+    test("constructor with single numeric argument", () => {
+        var a = new Array(5);
+        expect(a instanceof Array).toBeTrue();
+        expect(a).toHaveLength(5);
+    });
+
+    test("constructor with single non-numeric argument", () => {
+        var a = new Array("5");
+        expect(a instanceof Array).toBeTrue();
+        expect(a).toHaveLength(1);
+        expect(a[0]).toBe("5");
+    });
+
+    test("constructor with multiple numeric arguments", () => {
+        var a = new Array(1, 2, 3);
+        expect(a instanceof Array).toBeTrue();
+        expect(a).toHaveLength(3);
+        expect(a[0]).toBe(1);
+        expect(a[1]).toBe(2);
+        expect(a[2]).toBe(3);
+    });
+
+    test("constructor with single array argument", () => {
+        var a = new Array([1, 2, 3]);
+        expect(a instanceof Array).toBeTrue();
+        expect(a).toHaveLength(1);
+        expect(a[0][0]).toBe(1);
+        expect(a[0][1]).toBe(2);
+        expect(a[0][2]).toBe(3);
+    });
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Array/Array.of.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Array/Array.of.js
@ -0,0 +1,59 @@
+test("length is 0", () => {
+    expect(Array.of).toHaveLength(0);
+});
+
+describe("normal behavior", () => {
+    test("single numeric argument", () => {
+        var a = Array.of(5);
+        expect(a instanceof Array).toBeTrue();
+        expect(a).toHaveLength(1);
+        expect(a[0]).toBe(5);
+    });
+
+    test("single non-numeric argument", () => {
+        var a = Array.of("5");
+        expect(a instanceof Array).toBeTrue();
+        expect(a).toHaveLength(1);
+        expect(a[0]).toBe("5");
+    });
+
+    test("single infinite numeric argument", () => {
+        var a = Array.of(Infinity);
+        expect(a instanceof Array).toBeTrue();
+        expect(a).toHaveLength(1);
+        expect(a[0]).toBe(Infinity);
+    });
+
+    test("multiple numeric arguments", () => {
+        var a = Array.of(1, 2, 3);
+        expect(a instanceof Array).toBeTrue();
+        expect(a).toHaveLength(3);
+        expect(a[0]).toBe(1);
+        expect(a[1]).toBe(2);
+        expect(a[2]).toBe(3);
+    });
+
+    test("single array argument", () => {
+        var a = Array.of([1, 2, 3]);
+        expect(a instanceof Array).toBeTrue();
+        expect(a).toHaveLength(1);
+        expect(a[0][0]).toBe(1);
+        expect(a[0][1]).toBe(2);
+        expect(a[0][2]).toBe(3);
+    });
+
+    test("getter property is included in returned array", () => {
+        var t = [1, 2, 3];
+        Object.defineProperty(t, 3, {
+            get() {
+                return 4;
+            },
+        });
+        var a = Array.of(...t);
+        expect(a).toHaveLength(4);
+        expect(a[0]).toBe(1);
+        expect(a[1]).toBe(2);
+        expect(a[2]).toBe(3);
+        expect(a[3]).toBe(4);
+    });
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype-generic-functions.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype-generic-functions.js
@ -0,0 +1,150 @@
+describe("ability to work with generic non-array objects", () => {
+    test("push, pop", () => {
+        [undefined, "foo", -42, 0].forEach(length => {
+            const o = { length };
+
+            expect(Array.prototype.push.call(o, "foo")).toBe(1);
+            expect(o).toHaveLength(1);
+            expect(o[0]).toBe("foo");
+            expect(Array.prototype.push.call(o, "bar", "baz")).toBe(3);
+            expect(o).toHaveLength(3);
+            expect(o[0]).toBe("foo");
+            expect(o[1]).toBe("bar");
+            expect(o[2]).toBe("baz");
+
+            expect(Array.prototype.pop.call(o)).toBe("baz");
+            expect(o).toHaveLength(2);
+            expect(Array.prototype.pop.call(o)).toBe("bar");
+            expect(o).toHaveLength(1);
+            expect(Array.prototype.pop.call(o)).toBe("foo");
+            expect(o).toHaveLength(0);
+            expect(Array.prototype.pop.call(o)).toBeUndefined();
+            expect(o).toHaveLength(0);
+
+            o.length = length;
+            expect(Array.prototype.pop.call(o)).toBeUndefined();
+            expect(o).toHaveLength(0);
+        });
+    });
+
+    test("splice", () => {
+        const o = { length: 3, 0: "hello", 2: "serenity" };
+        const removed = Array.prototype.splice.call(o, 0, 2, "hello", "friends");
+        expect(o).toHaveLength(3);
+        expect(o[0]).toBe("hello");
+        expect(o[1]).toBe("friends");
+        expect(o[2]).toBe("serenity");
+        expect(removed).toHaveLength(2);
+        expect(removed[0]).toBe("hello");
+        expect(removed[1]).toBeUndefined();
+    });
+
+    test("join", () => {
+        expect(Array.prototype.join.call({})).toBe("");
+        expect(Array.prototype.join.call({ length: "foo" })).toBe("");
+        expect(Array.prototype.join.call({ length: 3 })).toBe(",,");
+        expect(Array.prototype.join.call({ length: 2, 0: "foo", 1: "bar" })).toBe("foo,bar");
+        expect(Array.prototype.join.call({ length: 2, 0: "foo", 1: "bar", 2: "baz" })).toBe(
+            "foo,bar"
+        );
+        expect(Array.prototype.join.call({ length: 3, 1: "bar" }, "~")).toBe("~bar~");
+        expect(Array.prototype.join.call({ length: 3, 0: "foo", 1: "bar", 2: "baz" }, "~")).toBe(
+            "foo~bar~baz"
+        );
+    });
+
+    // FIXME: test-js asserts when this is just called "toString" ಠ_ಠ
+    test("toString (FIXME)", () => {
+        expect(Array.prototype.toString.call({})).toBe("[object Object]");
+        expect(Array.prototype.toString.call({ join: "foo" })).toBe("[object Object]");
+        expect(Array.prototype.toString.call({ join: () => "foo" })).toBe("foo");
+    });
+
+    test("indexOf", () => {
+        expect(Array.prototype.indexOf.call({})).toBe(-1);
+        expect(Array.prototype.indexOf.call({ 0: undefined })).toBe(-1);
+        expect(Array.prototype.indexOf.call({ length: 1, 0: undefined })).toBe(0);
+        expect(Array.prototype.indexOf.call({ length: 1, 2: "foo" }, "foo")).toBe(-1);
+        expect(Array.prototype.indexOf.call({ length: 5, 2: "foo" }, "foo")).toBe(2);
+        expect(Array.prototype.indexOf.call({ length: 5, 2: "foo", 4: "foo" }, "foo", 3)).toBe(4);
+    });
+
+    test("lastIndexOf", () => {
+        expect(Array.prototype.lastIndexOf.call({})).toBe(-1);
+        expect(Array.prototype.lastIndexOf.call({ 0: undefined })).toBe(-1);
+        expect(Array.prototype.lastIndexOf.call({ length: 1, 0: undefined })).toBe(0);
+        expect(Array.prototype.lastIndexOf.call({ length: 1, 2: "foo" }, "foo")).toBe(-1);
+        expect(Array.prototype.lastIndexOf.call({ length: 5, 2: "foo" }, "foo")).toBe(2);
+        expect(Array.prototype.lastIndexOf.call({ length: 5, 2: "foo", 4: "foo" }, "foo")).toBe(4);
+        expect(Array.prototype.lastIndexOf.call({ length: 5, 2: "foo", 4: "foo" }, "foo", -2)).toBe(
+            2
+        );
+    });
+
+    test("includes", () => {
+        expect(Array.prototype.includes.call({})).toBeFalse();
+        expect(Array.prototype.includes.call({ 0: undefined })).toBeFalse();
+        expect(Array.prototype.includes.call({ length: 1, 0: undefined })).toBeTrue();
+        expect(Array.prototype.includes.call({ length: 1, 2: "foo" }, "foo")).toBeFalse();
+        expect(Array.prototype.includes.call({ length: 5, 2: "foo" }, "foo")).toBeTrue();
+    });
+
+    const o = { length: 5, 0: "foo", 1: "bar", 3: "baz" };
+
+    test("every", () => {
+        const visited = [];
+        Array.prototype.every.call(o, value => {
+            visited.push(value);
+            return true;
+        });
+        expect(visited).toEqual(["foo", "bar", "baz"]);
+    });
+
+    test("find, findIndex", () => {
+        ["find", "findIndex"].forEach(name => {
+            const visited = [];
+            Array.prototype[name].call(o, value => {
+                visited.push(value);
+                return false;
+            });
+            expect(visited).toEqual(["foo", "bar", undefined, "baz", undefined]);
+        });
+    });
+
+    test("filter, forEach, map, some", () => {
+        ["filter", "forEach", "map", "some"].forEach(name => {
+            const visited = [];
+            Array.prototype[name].call(o, value => {
+                visited.push(value);
+                return false;
+            });
+            expect(visited).toEqual(["foo", "bar", "baz"]);
+        });
+    });
+
+    test("reduce", () => {
+        const visited = [];
+        Array.prototype.reduce.call(
+            o,
+            (_, value) => {
+                visited.push(value);
+                return false;
+            },
+            "initial"
+        );
+        expect(visited).toEqual(["foo", "bar", "baz"]);
+    });
+
+    test("reduceRight", () => {
+        const visited = [];
+        Array.prototype.reduceRight.call(
+            o,
+            (_, value) => {
+                visited.push(value);
+                return false;
+            },
+            "initial"
+        );
+        expect(visited).toEqual(["baz", "bar", "foo"]);
+    });
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.concat.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.concat.js
@ -0,0 +1,43 @@
+test("length is 1", () => {
+    expect(Array.prototype.concat).toHaveLength(1);
+});
+
+describe("normal behavior", () => {
+    var array = ["hello"];
+
+    test("no arguments", () => {
+        var concatenated = array.concat();
+        expect(array).toHaveLength(1);
+        expect(concatenated).toHaveLength(1);
+    });
+
+    test("single argument", () => {
+        var concatenated = array.concat("friends");
+        expect(array).toHaveLength(1);
+        expect(concatenated).toHaveLength(2);
+        expect(concatenated[0]).toBe("hello");
+        expect(concatenated[1]).toBe("friends");
+    });
+
+    test("single array argument", () => {
+        var concatenated = array.concat([1, 2, 3]);
+        expect(array).toHaveLength(1);
+        expect(concatenated).toHaveLength(4);
+        expect(concatenated[0]).toBe("hello");
+        expect(concatenated[1]).toBe(1);
+        expect(concatenated[2]).toBe(2);
+        expect(concatenated[3]).toBe(3);
+    });
+
+    test("multiple arguments", () => {
+        var concatenated = array.concat(false, "serenity", { name: "libjs" }, [1, [2, 3]]);
+        expect(array).toHaveLength(1);
+        expect(concatenated).toHaveLength(6);
+        expect(concatenated[0]).toBe("hello");
+        expect(concatenated[1]).toBeFalse();
+        expect(concatenated[2]).toBe("serenity");
+        expect(concatenated[3]).toEqual({ name: "libjs" });
+        expect(concatenated[4]).toBe(1);
+        expect(concatenated[5]).toEqual([2, 3]);
+    });
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.every.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.every.js
@ -0,0 +1,55 @@
+test("length is 1", () => {
+    expect(Array.prototype.every).toHaveLength(1);
+});
+
+describe("errors", () => {
+    test("requires at least one argument", () => {
+        expect(() => {
+            [].every();
+        }).toThrowWithMessage(TypeError, "Array.prototype.every() requires at least one argument");
+    });
+
+    test("callback must be a function", () => {
+        expect(() => {
+            [].every(undefined);
+        }).toThrowWithMessage(TypeError, "undefined is not a function");
+    });
+});
+
+describe("normal behavior", () => {
+    test("basic functionality", () => {
+        var arrayOne = ["serenity", { test: "serenity" }];
+        var arrayTwo = [true, false, 1, 2, 3, "3"];
+
+        expect(arrayOne.every(value => value === "hello")).toBeFalse();
+        expect(arrayOne.every(value => value === "serenity")).toBeFalse();
+        expect(arrayOne.every((value, index, arr) => index < 2)).toBeTrue();
+        expect(arrayOne.every(value => typeof value === "string")).toBeFalse();
+        expect(arrayOne.every(value => arrayOne.pop())).toBeTrue();
+
+        expect(arrayTwo.every((value, index, arr) => index > 0)).toBeFalse();
+        expect(arrayTwo.every((value, index, arr) => index >= 0)).toBeTrue();
+        expect(arrayTwo.every(value => typeof value !== "string")).toBeFalse();
+        expect(arrayTwo.every(value => typeof value === "number")).toBeFalse();
+        expect(arrayTwo.every(value => value > 0)).toBeFalse();
+        expect(arrayTwo.every(value => value >= 0 && value < 4)).toBeTrue();
+        expect(arrayTwo.every(value => arrayTwo.pop())).toBeTrue();
+
+        expect(["", "hello", "friends", "serenity"].every(value => value.length >= 0)).toBeTrue();
+    });
+
+    test("empty array", () => {
+        expect([].every(value => value === 1)).toBeTrue();
+    });
+
+    test("elements past the initial array size are ignored", () => {
+        var array = [1, 2, 3, 4, 5];
+
+        expect(
+            arrayTwo.every((value, index, arr) => {
+                arr.push(6);
+                return value <= 5;
+            })
+        ).toBeTrue();
+    });
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.fill.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.fill.js
@ -0,0 +1,20 @@
+test("length is 1", () => {
+    expect(Array.prototype.fill).toHaveLength(1);
+});
+
+test("basic functionality", () => {
+    var array = [1, 2, 3, 4];
+
+    expect(array.fill(0, 2, 4)).toEqual([1, 2, 0, 0]);
+    expect(array.fill(5, 1)).toEqual([1, 5, 5, 5]);
+    expect(array.fill(6)).toEqual([6, 6, 6, 6]);
+
+    expect([1, 2, 3].fill(4)).toEqual([4, 4, 4]);
+    expect([1, 2, 3].fill(4, 1)).toEqual([1, 4, 4]);
+    expect([1, 2, 3].fill(4, 1, 2)).toEqual([1, 4, 3]);
+    expect([1, 2, 3].fill(4, 3, 3)).toEqual([1, 2, 3]);
+    expect([1, 2, 3].fill(4, -3, -2)).toEqual([4, 2, 3]);
+    expect([1, 2, 3].fill(4, NaN, NaN)).toEqual([1, 2, 3]);
+    expect([1, 2, 3].fill(4, 3, 5)).toEqual([1, 2, 3]);
+    expect(Array(3).fill(4)).toEqual([4, 4, 4]);
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.filter.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.filter.js
@ -0,0 +1,68 @@
+test("length is 1", () => {
+    expect(Array.prototype.filter).toHaveLength(1);
+});
+
+describe("errors", () => {
+    test("requires at least one argument", () => {
+        expect(() => {
+            [].filter();
+        }).toThrowWithMessage(TypeError, "Array.prototype.filter() requires at least one argument");
+    });
+
+    test("callback must be a function", () => {
+        expect(() => {
+            [].filter(undefined);
+        }).toThrowWithMessage(TypeError, "undefined is not a function");
+    });
+});
+
+describe("normal behavior", () => {
+    test("never calls callback with empty array", () => {
+        var callbackCalled = 0;
+        expect(
+            [].filter(() => {
+                callbackCalled++;
+            })
+        ).toEqual([]);
+        expect(callbackCalled).toBe(0);
+    });
+
+    test("calls callback once for every item", () => {
+        var callbackCalled = 0;
+        expect(
+            [1, 2, 3].filter(() => {
+                callbackCalled++;
+            })
+        ).toEqual([]);
+        expect(callbackCalled).toBe(3);
+    });
+
+    test("can filter based on callback return value", () => {
+        var evenNumbers = [0, 1, 2, 3, 4, 5, 6, 7].filter(x => x % 2 === 0);
+        expect(evenNumbers).toEqual([0, 2, 4, 6]);
+
+        var fruits = [
+            "Apple",
+            "Banana",
+            "Blueberry",
+            "Grape",
+            "Mango",
+            "Orange",
+            "Peach",
+            "Pineapple",
+            "Raspberry",
+            "Watermelon",
+        ];
+        const filterItems = (arr, query) => {
+            return arr.filter(el => el.toLowerCase().indexOf(query.toLowerCase()) !== -1);
+        };
+        expect(filterItems(fruits, "Berry")).toEqual(["Blueberry", "Raspberry"]);
+        expect(filterItems(fruits, "P")).toEqual([
+            "Apple",
+            "Grape",
+            "Peach",
+            "Pineapple",
+            "Raspberry",
+        ]);
+    });
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.find.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.find.js
@ -0,0 +1,65 @@
+test("length is 1", () => {
+    expect(Array.prototype.find).toHaveLength(1);
+});
+
+describe("errors", () => {
+    test("requires at least one argument", () => {
+        expect(() => {
+            [].find();
+        }).toThrowWithMessage(TypeError, "Array.prototype.find() requires at least one argument");
+    });
+
+    test("callback must be a function", () => {
+        expect(() => {
+            [].find(undefined);
+        }).toThrowWithMessage(TypeError, "undefined is not a function");
+    });
+});
+
+describe("normal behavior", () => {
+    test("basic functionality", () => {
+        var array = ["hello", "friends", 1, 2, false];
+
+        expect(array.find(value => value === "hello")).toBe("hello");
+        expect(array.find((value, index, arr) => index === 1)).toBe("friends");
+        expect(array.find(value => value == "1")).toBe(1);
+        expect(array.find(value => value === 1)).toBe(1);
+        expect(array.find(value => typeof value !== "string")).toBe(1);
+        expect(array.find(value => typeof value === "boolean")).toBeFalse();
+        expect(array.find(value => value > 1)).toBe(2);
+        expect(array.find(value => value > 1 && value < 3)).toBe(2);
+        expect(array.find(value => value > 100)).toBeUndefined();
+        expect([].find(value => value === 1)).toBeUndefined();
+    });
+
+    test("never calls callback with empty array", () => {
+        var callbackCalled = 0;
+        expect(
+            [].find(() => {
+                callbackCalled++;
+            })
+        ).toBeUndefined();
+        expect(callbackCalled).toBe(0);
+    });
+
+    test("calls callback once for every item", () => {
+        var callbackCalled = 0;
+        expect(
+            [1, 2, 3].find(() => {
+                callbackCalled++;
+            })
+        ).toBeUndefined();
+        expect(callbackCalled).toBe(3);
+    });
+
+    test("empty slots are treated as undefined", () => {
+        var callbackCalled = 0;
+        expect(
+            [1, , , "foo", , undefined, , ,].find(value => {
+                callbackCalled++;
+                return value === undefined;
+            })
+        ).toBeUndefined();
+        expect(callbackCalled).toBe(2);
+    });
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.findIndex.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.findIndex.js
@ -0,0 +1,68 @@
+test("length is 1", () => {
+    expect(Array.prototype.findIndex).toHaveLength(1);
+});
+
+describe("errors", () => {
+    test("requires at least one argument", () => {
+        expect(() => {
+            [].findIndex();
+        }).toThrowWithMessage(
+            TypeError,
+            "Array.prototype.findIndex() requires at least one argument"
+        );
+    });
+
+    test("callback must be a function", () => {
+        expect(() => {
+            [].findIndex(undefined);
+        }).toThrowWithMessage(TypeError, "undefined is not a function");
+    });
+});
+
+describe("normal behavior", () => {
+    test("basic functionality", () => {
+        var array = ["hello", "friends", 1, 2, false];
+
+        expect(array.findIndex(value => value === "hello")).toBe(0);
+        expect(array.findIndex((value, index, arr) => index === 1)).toBe(1);
+        expect(array.findIndex(value => value == "1")).toBe(2);
+        expect(array.findIndex(value => value === 1)).toBe(2);
+        expect(array.findIndex(value => typeof value !== "string")).toBe(2);
+        expect(array.findIndex(value => typeof value === "boolean")).toBe(4);
+        expect(array.findIndex(value => value > 1)).toBe(3);
+        expect(array.findIndex(value => value > 1 && value < 3)).toBe(3);
+        expect(array.findIndex(value => value > 100)).toBe(-1);
+        expect([].findIndex(value => value === 1)).toBe(-1);
+    });
+
+    test("never calls callback with empty array", () => {
+        var callbackCalled = 0;
+        expect(
+            [].findIndex(() => {
+                callbackCalled++;
+            })
+        ).toBe(-1);
+        expect(callbackCalled).toBe(0);
+    });
+
+    test("calls callback once for every item", () => {
+        var callbackCalled = 0;
+        expect(
+            [1, 2, 3].findIndex(() => {
+                callbackCalled++;
+            })
+        ).toBe(-1);
+        expect(callbackCalled).toBe(3);
+    });
+
+    test("empty slots are treated as undefined", () => {
+        var callbackCalled = 0;
+        expect(
+            [1, , , "foo", , undefined, , ,].findIndex(value => {
+                callbackCalled++;
+                return value === undefined;
+            })
+        ).toBe(1);
+        expect(callbackCalled).toBe(2);
+    });
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.forEach.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.forEach.js
@ -0,0 +1,70 @@
+test("length is 1", () => {
+    expect(Array.prototype.forEach).toHaveLength(1);
+});
+
+describe("errors", () => {
+    test("requires at least one argument", () => {
+        expect(() => {
+            [].forEach();
+        }).toThrowWithMessage(
+            TypeError,
+            "Array.prototype.forEach() requires at least one argument"
+        );
+    });
+
+    test("callback must be a function", () => {
+        expect(() => {
+            [].forEach(undefined);
+        }).toThrowWithMessage(TypeError, "undefined is not a function");
+    });
+});
+
+describe("normal behavior", () => {
+    test("never calls callback with empty array", () => {
+        var callbackCalled = 0;
+        expect(
+            [].forEach(() => {
+                callbackCalled++;
+            })
+        ).toBeUndefined();
+        expect(callbackCalled).toBe(0);
+    });
+
+    test("calls callback once for every item", () => {
+        var callbackCalled = 0;
+        expect(
+            [1, 2, 3].forEach(() => {
+                callbackCalled++;
+            })
+        ).toBeUndefined();
+        expect(callbackCalled).toBe(3);
+    });
+
+    test("callback receives value and index", () => {
+        var a = [1, 2, 3];
+        a.forEach((value, index) => {
+            expect(value).toBe(a[index]);
+            expect(index).toBe(a[index] - 1);
+        });
+    });
+
+    test("callback receives array", () => {
+        var callbackCalled = 0;
+        var a = [1, 2, 3];
+        a.forEach((_, __, array) => {
+            callbackCalled++;
+            expect(a).toEqual(array);
+            a.push("test");
+        });
+        expect(callbackCalled).toBe(3);
+        expect(a).toEqual([1, 2, 3, "test", "test", "test"]);
+    });
+
+    test("this value can be modified", () => {
+        var t = [];
+        [1, 2, 3].forEach(function (value) {
+            this.push(value);
+        }, t);
+        expect(t).toEqual([1, 2, 3]);
+    });
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.includes.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.includes.js
@ -0,0 +1,18 @@
+test("length is 1", () => {
+    expect(Array.prototype.includes).toHaveLength(1);
+});
+
+test("basic functionality", () => {
+    var array = ["hello", "friends", 1, 2, false];
+
+    expect([].includes()).toBeFalse();
+    expect([undefined].includes()).toBeTrue();
+    expect(array.includes("hello")).toBeTrue();
+    expect(array.includes(1)).toBeTrue();
+    expect(array.includes(1, -3)).toBeTrue();
+    expect(array.includes("serenity")).toBeFalse();
+    expect(array.includes(false, -1)).toBeTrue();
+    expect(array.includes(2, -1)).toBeFalse();
+    expect(array.includes(2, -100)).toBeTrue();
+    expect(array.includes("friends", 100)).toBeFalse();
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.indexOf.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.indexOf.js
@ -0,0 +1,25 @@
+test("length is 1", () => {
+    expect(Array.prototype.indexOf).toHaveLength(1);
+});
+
+test("basic functionality", () => {
+    var array = ["hello", "friends", 1, 2, false];
+
+    expect(array.indexOf("hello")).toBe(0);
+    expect(array.indexOf("friends")).toBe(1);
+    expect(array.indexOf(false)).toBe(4);
+    expect(array.indexOf(false, 2)).toBe(4);
+    expect(array.indexOf(false, -2)).toBe(4);
+    expect(array.indexOf(1)).toBe(2);
+    expect(array.indexOf(1, 1000)).toBe(-1);
+    expect(array.indexOf(1, -1000)).toBe(2);
+    expect(array.indexOf("serenity")).toBe(-1);
+    expect(array.indexOf(false, -1)).toBe(4);
+    expect(array.indexOf(2, -1)).toBe(-1);
+    expect(array.indexOf(2, -2)).toBe(3);
+    expect([].indexOf("serenity")).toBe(-1);
+    expect([].indexOf("serenity", 10)).toBe(-1);
+    expect([].indexOf("serenity", -10)).toBe(-1);
+    expect([].indexOf()).toBe(-1);
+    expect([undefined].indexOf()).toBe(0);
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.join.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.join.js
@ -0,0 +1,24 @@
+test("length is 1", () => {
+    expect(Array.prototype.join).toHaveLength(1);
+});
+
+test("basic functionality", () => {
+    expect(["hello", "friends"].join()).toBe("hello,friends");
+    expect(["hello", "friends"].join(undefined)).toBe("hello,friends");
+    expect(["hello", "friends"].join(" ")).toBe("hello friends");
+    expect(["hello", "friends", "foo"].join("~", "#")).toBe("hello~friends~foo");
+    expect([].join()).toBe("");
+    expect([null].join()).toBe("");
+    expect([undefined].join()).toBe("");
+    expect([undefined, null, ""].join()).toBe(",,");
+    expect([1, null, 2, undefined, 3].join()).toBe("1,,2,,3");
+    expect(Array(3).join()).toBe(",,");
+});
+
+test("circular references", () => {
+    const a = ["foo", [], [1, 2, []], ["bar"]];
+    a[1] = a;
+    a[2][2] = a;
+    // [ "foo", <circular>, [ 1, 2, <circular> ], [ "bar" ] ]
+    expect(a.join()).toBe("foo,,1,2,,bar");
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.lastIndexOf.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.lastIndexOf.js
@ -0,0 +1,22 @@
+test("length is 1", () => {
+    expect(Array.prototype.lastIndexOf).toHaveLength(1);
+});
+
+test("basic functionality", () => {
+    var array = [1, 2, 3, 1, "hello"];
+
+    expect(array.lastIndexOf("hello")).toBe(4);
+    expect(array.lastIndexOf("hello", 1000)).toBe(4);
+    expect(array.lastIndexOf(1)).toBe(3);
+    expect(array.lastIndexOf(1, -1)).toBe(3);
+    expect(array.lastIndexOf(1, -2)).toBe(3);
+    expect(array.lastIndexOf(2)).toBe(1);
+    expect(array.lastIndexOf(2, -3)).toBe(1);
+    expect(array.lastIndexOf(2, -4)).toBe(1);
+    expect([].lastIndexOf("hello")).toBe(-1);
+    expect([].lastIndexOf("hello", 10)).toBe(-1);
+    expect([].lastIndexOf("hello", -10)).toBe(-1);
+    expect([].lastIndexOf()).toBe(-1);
+    expect([undefined].lastIndexOf()).toBe(0);
+    expect([undefined, undefined, undefined].lastIndexOf()).toBe(2);
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.map.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.map.js
@ -0,0 +1,57 @@
+test("length is 1", () => {
+    expect(Array.prototype.map).toHaveLength(1);
+});
+
+describe("errors", () => {
+    test("requires at least one argument", () => {
+        expect(() => {
+            [].map();
+        }).toThrowWithMessage(TypeError, "Array.prototype.map() requires at least one argument");
+    });
+
+    test("callback must be a function", () => {
+        expect(() => {
+            [].map(undefined);
+        }).toThrowWithMessage(TypeError, "undefined is not a function");
+    });
+});
+
+describe("normal behavior", () => {
+    test("never calls callback with empty array", () => {
+        var callbackCalled = 0;
+        expect(
+            [].map(() => {
+                callbackCalled++;
+            })
+        ).toEqual([]);
+        expect(callbackCalled).toBe(0);
+    });
+
+    test("calls callback once for every item", () => {
+        var callbackCalled = 0;
+        expect(
+            [1, 2, 3].map(() => {
+                callbackCalled++;
+            })
+        ).toEqual([undefined, undefined, undefined]);
+        expect(callbackCalled).toBe(3);
+    });
+
+    test("can map based on callback return value", () => {
+        expect(
+            [undefined, null, true, "foo", 42, {}].map(
+                (value, index) => "" + index + " -> " + value
+            )
+        ).toEqual([
+            "0 -> undefined",
+            "1 -> null",
+            "2 -> true",
+            "3 -> foo",
+            "4 -> 42",
+            "5 -> [object Object]",
+        ]);
+
+        var squaredNumbers = [0, 1, 2, 3, 4].map(x => x ** 2);
+        expect(squaredNumbers).toEqual([0, 1, 4, 9, 16]);
+    });
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.pop.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.pop.js
@ -0,0 +1,29 @@
+test("length is 0", () => {
+    expect(Array.prototype.pop).toHaveLength(0);
+});
+
+describe("normal behavior", () => {
+    test("array with elements", () => {
+        var a = [1, 2, 3];
+        expect(a.pop()).toBe(3);
+        expect(a).toEqual([1, 2]);
+        expect(a.pop()).toBe(2);
+        expect(a).toEqual([1]);
+        expect(a.pop()).toBe(1);
+        expect(a).toEqual([]);
+        expect(a.pop()).toBeUndefined();
+        expect(a).toEqual([]);
+    });
+
+    test("empty array", () => {
+        var a = [];
+        expect(a.pop()).toBeUndefined();
+        expect(a).toEqual([]);
+    });
+
+    test("array with empty slot", () => {
+        var a = [,];
+        expect(a.pop()).toBeUndefined();
+        expect(a).toEqual([]);
+    });
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.push.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.push.js
@ -0,0 +1,23 @@
+test("length is 1", () => {
+    expect(Array.prototype.push).toHaveLength(1);
+});
+
+describe("normal behavior", () => {
+    test("no argument", () => {
+        var a = ["hello"];
+        expect(a.push()).toBe(1);
+        expect(a).toEqual(["hello"]);
+    });
+
+    test("single argument", () => {
+        var a = ["hello"];
+        expect(a.push("friends")).toBe(2);
+        expect(a).toEqual(["hello", "friends"]);
+    });
+
+    test("multiple arguments", () => {
+        var a = ["hello", "friends"];
+        expect(a.push(1, 2, 3)).toBe(5);
+        expect(a).toEqual(["hello", "friends", 1, 2, 3]);
+    });
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.reduce.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.reduce.js
@ -0,0 +1,113 @@
+test("length is 1", () => {
+    expect(Array.prototype.reduce).toHaveLength(1);
+});
+
+describe("errors", () => {
+    test("requires at least one argument", () => {
+        expect(() => {
+            [].reduce();
+        }).toThrowWithMessage(TypeError, "Array.prototype.reduce() requires at least one argument");
+    });
+
+    test("callback must be a function", () => {
+        expect(() => {
+            [].reduce(undefined);
+        }).toThrowWithMessage(TypeError, "undefined is not a function");
+    });
+
+    test("reduce of empty array with no initial value", () => {
+        expect(() => {
+            [].reduce((a, x) => x);
+        }).toThrowWithMessage(TypeError, "Reduce of empty array with no initial value");
+    });
+
+    test("reduce of array with only empty slots and no initial value", () => {
+        expect(() => {
+            [, ,].reduce((a, x) => x);
+        }).toThrowWithMessage(TypeError, "Reduce of empty array with no initial value");
+    });
+});
+
+describe("normal behavior", () => {
+    test("basic functionality", () => {
+        [1, 2].reduce(function () {
+            expect(this).toBeUndefined();
+        });
+
+        var callbackCalled = 0;
+        var callback = () => {
+            callbackCalled++;
+            return true;
+        };
+
+        expect([1].reduce(callback)).toBe(1);
+        expect(callbackCalled).toBe(0);
+
+        expect([, 1].reduce(callback)).toBe(1);
+        expect(callbackCalled).toBe(0);
+
+        callbackCalled = 0;
+        expect([1, 2, 3].reduce(callback)).toBeTrue();
+        expect(callbackCalled).toBe(2);
+
+        callbackCalled = 0;
+        expect([, , 1, 2, 3].reduce(callback)).toBeTrue();
+        expect(callbackCalled).toBe(2);
+
+        callbackCalled = 0;
+        expect([1, , , 10, , 100, , ,].reduce(callback)).toBeTrue();
+        expect(callbackCalled).toBe(2);
+
+        var constantlySad = () => ":^(";
+        var result = [].reduce(constantlySad, ":^)");
+        expect(result).toBe(":^)");
+
+        result = [":^0"].reduce(constantlySad, ":^)");
+        expect(result).toBe(":^(");
+
+        result = [":^0"].reduce(constantlySad);
+        expect(result).toBe(":^0");
+
+        result = [5, 4, 3, 2, 1].reduce((accum, elem) => accum + elem);
+        expect(result).toBe(15);
+
+        result = [1, 2, 3, 4, 5, 6].reduce((accum, elem) => accum + elem, 100);
+        expect(result).toBe(121);
+
+        result = [6, 5, 4, 3, 2, 1].reduce((accum, elem) => {
+            return accum + elem;
+        }, 100);
+        expect(result).toBe(121);
+
+        var indexes = [];
+        result = ["foo", 1, true].reduce((a, v, i) => {
+            indexes.push(i);
+        });
+        expect(result).toBeUndefined();
+        expect(indexes.length).toBe(2);
+        expect(indexes[0]).toBe(1);
+        expect(indexes[1]).toBe(2);
+
+        indexes = [];
+        result = ["foo", 1, true].reduce((a, v, i) => {
+            indexes.push(i);
+        }, "foo");
+        expect(result).toBeUndefined();
+        expect(indexes).toEqual([0, 1, 2]);
+
+        var mutable = { prop: 0 };
+        result = ["foo", 1, true].reduce((a, v) => {
+            a.prop = v;
+            return a;
+        }, mutable);
+        expect(result).toBe(mutable);
+        expect(result.prop).toBeTrue();
+
+        var a1 = [1, 2];
+        var a2 = null;
+        a1.reduce((a, v, i, t) => {
+            a2 = t;
+        });
+        expect(a1).toBe(a2);
+    });
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.reduceRight.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.reduceRight.js
@ -0,0 +1,118 @@
+test("length is 1", () => {
+    expect(Array.prototype.reduceRight).toHaveLength(1);
+});
+
+describe("errors", () => {
+    test("requires at least one argument", () => {
+        expect(() => {
+            [].reduceRight();
+        }).toThrowWithMessage(
+            TypeError,
+            "Array.prototype.reduceRight() requires at least one argument"
+        );
+    });
+
+    test("callback must be a function", () => {
+        expect(() => {
+            [].reduceRight(undefined);
+        }).toThrowWithMessage(TypeError, "undefined is not a function");
+    });
+
+    test("reduce of empty array with no initial value", () => {
+        expect(() => {
+            [].reduceRight((a, x) => x);
+        }).toThrowWithMessage(TypeError, "Reduce of empty array with no initial value");
+    });
+
+    test("reduce of array with only empty slots and no initial value", () => {
+        expect(() => {
+            [, ,].reduceRight((a, x) => x);
+        }).toThrowWithMessage(TypeError, "Reduce of empty array with no initial value");
+    });
+});
+
+describe("normal behavior", () => {
+    test("basic functionality", () => {
+        [1, 2].reduceRight(function () {
+            expect(this).toBeUndefined();
+        });
+
+        var callbackCalled = 0;
+        var callback = () => {
+            callbackCalled++;
+            return true;
+        };
+
+        expect([1].reduceRight(callback)).toBe(1);
+        expect(callbackCalled).toBe(0);
+
+        expect([1].reduceRight(callback)).toBe(1);
+        expect(callbackCalled).toBe(0);
+
+        callbackCalled = 0;
+        expect([1, 2, 3].reduceRight(callback)).toBe(true);
+        expect(callbackCalled).toBe(2);
+
+        callbackCalled = 0;
+        expect([1, 2, 3, ,].reduceRight(callback)).toBe(true);
+        expect(callbackCalled).toBe(2);
+
+        callbackCalled = 0;
+        expect([, , , 1, , , 10, , 100, , ,].reduceRight(callback)).toBe(true);
+        expect(callbackCalled).toBe(2);
+
+        var constantlySad = () => ":^(";
+        var result = [].reduceRight(constantlySad, ":^)");
+        expect(result).toBe(":^)");
+
+        result = [":^0"].reduceRight(constantlySad, ":^)");
+        expect(result).toBe(":^(");
+
+        result = [":^0"].reduceRight(constantlySad);
+        expect(result).toBe(":^0");
+
+        result = [5, 4, 3, 2, 1].reduceRight((accum, elem) => "" + accum + elem);
+        expect(result).toBe("12345");
+
+        result = [1, 2, 3, 4, 5, 6].reduceRight((accum, elem) => {
+            return "" + accum + elem;
+        }, 100);
+        expect(result).toBe("100654321");
+
+        result = [6, 5, 4, 3, 2, 1].reduceRight((accum, elem) => {
+            return "" + accum + elem;
+        }, 100);
+        expect(result).toBe("100123456");
+
+        var indexes = [];
+        result = ["foo", 1, true].reduceRight((a, v, i) => {
+            indexes.push(i);
+        });
+        expect(result).toBeUndefined();
+        expect(indexes.length).toBe(2);
+        expect(indexes[0]).toBe(1);
+        expect(indexes[1]).toBe(0);
+
+        indexes = [];
+        result = ["foo", 1, true].reduceRight((a, v, i) => {
+            indexes.push(i);
+        }, "foo");
+        expect(result).toBeUndefined();
+        expect(indexes).toEqual([2, 1, 0]);
+
+        var mutable = { prop: 0 };
+        result = ["foo", 1, true].reduceRight((a, v) => {
+            a.prop = v;
+            return a;
+        }, mutable);
+        expect(result).toBe(mutable);
+        expect(result.prop).toBe("foo");
+
+        var a1 = [1, 2];
+        var a2 = null;
+        a1.reduceRight((a, v, i, t) => {
+            a2 = t;
+        });
+        expect(a1).toBe(a2);
+    });
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.reverse.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.reverse.js
@ -0,0 +1,9 @@
+test("length is 0", () => {
+    expect(Array.prototype.reverse).toHaveLength(0);
+});
+
+test("basic functionality", () => {
+    var array = [1, 2, 3];
+    expect(array.reverse()).toEqual([3, 2, 1]);
+    expect(array).toEqual([3, 2, 1]);
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.shift.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.shift.js
@ -0,0 +1,23 @@
+test("length is 0", () => {
+    expect(Array.prototype.shift).toHaveLength(0);
+});
+
+describe("normal behavior", () => {
+    test("array with elements", () => {
+        var a = [1, 2, 3];
+        expect(a.shift()).toBe(1);
+        expect(a).toEqual([2, 3]);
+    });
+
+    test("empty array", () => {
+        var a = [];
+        expect(a.shift()).toBeUndefined();
+        expect(a).toEqual([]);
+    });
+
+    test("array with empty slot", () => {
+        var a = [,];
+        expect(a.shift()).toBeUndefined();
+        expect(a).toEqual([]);
+    });
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.slice.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.slice.js
@ -0,0 +1,39 @@
+test("length is 0", () => {
+    expect(Array.prototype.slice).toHaveLength(2);
+});
+
+test("basic functionality", () => {
+    var array = ["hello", "friends", "serenity", 1];
+
+    var slice = array.slice();
+    expect(array).toEqual(["hello", "friends", "serenity", 1]);
+    expect(slice).toEqual(["hello", "friends", "serenity", 1]);
+
+    slice = array.slice(1);
+    expect(array).toEqual(["hello", "friends", "serenity", 1]);
+    expect(slice).toEqual(["friends", "serenity", 1]);
+
+    slice = array.slice(0, 2);
+    expect(array).toEqual(["hello", "friends", "serenity", 1]);
+    expect(slice).toEqual(["hello", "friends"]);
+
+    slice = array.slice(-1);
+    expect(array).toEqual(["hello", "friends", "serenity", 1]);
+    expect(slice).toEqual([1]);
+
+    slice = array.slice(1, 1);
+    expect(array).toEqual(["hello", "friends", "serenity", 1]);
+    expect(slice).toEqual([]);
+
+    slice = array.slice(1, -1);
+    expect(array).toEqual(["hello", "friends", "serenity", 1]);
+    expect(slice).toEqual(["friends", "serenity"]);
+
+    slice = array.slice(2, -1);
+    expect(array).toEqual(["hello", "friends", "serenity", 1]);
+    expect(slice).toEqual(["serenity"]);
+
+    slice = array.slice(0, 100);
+    expect(array).toEqual(["hello", "friends", "serenity", 1]);
+    expect(slice).toEqual(["hello", "friends", "serenity", 1]);
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.some.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.some.js
@ -0,0 +1,37 @@
+test("length is 1", () => {
+    expect(Array.prototype.some).toHaveLength(1);
+});
+
+describe("errors", () => {
+    test("requires at least one argument", () => {
+        expect(() => {
+            [].some();
+        }).toThrowWithMessage(TypeError, "Array.prototype.some() requires at least one argument");
+    });
+
+    test("callback must be a function", () => {
+        expect(() => {
+            [].some(undefined);
+        }).toThrowWithMessage(TypeError, "undefined is not a function");
+    });
+});
+
+test("basic functionality", () => {
+    var array = ["hello", "friends", 1, 2, false, -42, { name: "serenityos" }];
+
+    expect(array.some(value => value === "hello")).toBeTrue();
+    expect(array.some(value => value === "serenity")).toBeFalse();
+    expect(array.some((value, index, arr) => index === 1)).toBeTrue();
+    expect(array.some(value => value == "1")).toBeTrue();
+    expect(array.some(value => value === 1)).toBeTrue();
+    expect(array.some(value => value === 13)).toBeFalse();
+    expect(array.some(value => typeof value !== "string")).toBeTrue();
+    expect(array.some(value => typeof value === "boolean")).toBeTrue();
+    expect(array.some(value => value > 1)).toBeTrue();
+    expect(array.some(value => value > 1 && value < 3)).toBeTrue();
+    expect(array.some(value => value > 100)).toBeFalse();
+    expect(array.some(value => value < 0)).toBeTrue();
+    expect(array.some(value => array.pop())).toBeTrue();
+    expect(["", "hello", "friends", "serenity"].some(value => value.length === 0)).toBeTrue();
+    expect([].some(value => value === 1)).toBeFalse();
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.sort.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.sort.js
@ -0,0 +1,204 @@
+describe("Array.prototype.sort", () => {
+    test("basic functionality", () => {
+        expect(Array.prototype.sort).toHaveLength(1);
+
+        var arr = ["c", "b", "d", "a"];
+        expect(arr.sort()).toEqual(arr);
+        expect(arr).toEqual(["a", "b", "c", "d"]);
+
+        arr = ["aa", "a"];
+        expect(arr.sort()).toEqual(arr);
+        expect(arr).toEqual(["a", "aa"]);
+
+        arr = [1, 0];
+        expect(arr.sort()).toBe(arr); // should be exactly same object
+        expect(arr).toEqual([0, 1]);
+
+        // numbers are sorted as strings
+        arr = [205, -123, 22, 200, 3, -20, -2, -1, 25, 2, 0, 1];
+        expect(arr.sort()).toEqual([-1, -123, -2, -20, 0, 1, 2, 200, 205, 22, 25, 3]);
+
+        // mix of data, including empty slots and undefined
+        arr = ["2", Infinity, null, null, , undefined, 5, , undefined, null, 54, "5"];
+        expect(arr.sort()).toEqual([
+            "2",
+            5,
+            "5",
+            54,
+            Infinity,
+            null,
+            null,
+            null,
+            undefined,
+            undefined,
+            ,
+            ,
+        ]);
+        expect(arr.length).toEqual(12);
+
+        // undefined compare function
+        arr = ["2", Infinity, null, null, , undefined, 5n, , undefined, null, 54, "5"];
+        expect(arr.sort(undefined)).toEqual([
+            "2",
+            5n,
+            "5",
+            54,
+            Infinity,
+            null,
+            null,
+            null,
+            undefined,
+            undefined,
+            ,
+            ,
+        ]);
+        expect(arr.length).toEqual(12);
+
+        // numeric data with compare function to sort numerically
+        arr = [50, 500, 5, Infinity, -Infinity, 0, 10, -10, 1, -1, 5, 0, 15, Infinity];
+        expect(arr.sort((a, b) => a - b)).toEqual([
+            -Infinity,
+            -10,
+            -1,
+            0,
+            0,
+            1,
+            5,
+            5,
+            10,
+            15,
+            50,
+            500,
+            Infinity,
+            Infinity,
+        ]);
+        expect(arr.length).toEqual(14);
+
+        // numeric data with compare function to sort reverse numerically
+        arr = [50, 500, 5, Infinity, -Infinity, 0, 10, -10, 1, -1, 5, 0, 15, Infinity];
+        expect(arr.sort((a, b) => b - a)).toEqual([
+            Infinity,
+            Infinity,
+            500,
+            50,
+            15,
+            10,
+            5,
+            5,
+            1,
+            0,
+            0,
+            -1,
+            -10,
+            -Infinity,
+        ]);
+
+        // small/edge cases
+        expect([].sort()).toEqual([]);
+        expect([5].sort()).toEqual([5]);
+        expect([5, 5].sort()).toEqual([5, 5]);
+        expect([undefined].sort()).toEqual([undefined]);
+        expect([undefined, undefined].sort()).toEqual([undefined, undefined]);
+        expect([,].sort()).toEqual([,]);
+        expect([, ,].sort()).toEqual([, ,]);
+        expect([5, ,].sort()).toEqual([5, ,]);
+        expect([, , 5].sort()).toEqual([5, , ,]);
+
+        // sorting should be stable
+        arr = [
+            { sorted_key: 2, other_property: 1 },
+            { sorted_key: 2, other_property: 2 },
+            { sorted_key: 1, other_property: 3 },
+        ];
+        arr.sort((a, b) => a.sorted_key - b.sorted_key);
+        expect(arr[1].other_property == 1);
+        expect(arr[2].other_property == 2);
+    });
+
+    test("that it makes no unnecessary calls to compare function", () => {
+        expectNoCallCompareFunction = function (a, b) {
+            expect().fail();
+        };
+
+        expect([].sort(expectNoCallCompareFunction)).toEqual([]);
+        expect([1].sort(expectNoCallCompareFunction)).toEqual([1]);
+        expect([1, undefined].sort(expectNoCallCompareFunction)).toEqual([1, undefined]);
+        expect([undefined, undefined].sort(expectNoCallCompareFunction)).toEqual([
+            undefined,
+            undefined,
+        ]);
+        expect([, , 1, ,].sort(expectNoCallCompareFunction)).toEqual([1, , , ,]);
+        expect([undefined, , 1, , undefined, ,].sort(expectNoCallCompareFunction)).toEqual([
+            1,
+            undefined,
+            undefined,
+            ,
+            ,
+            ,
+        ]);
+    });
+
+    test("that it works on non-arrays", () => {
+        var obj = { length: 0 };
+        expect(Array.prototype.sort.call(obj)).toBe(obj);
+        expect(obj).toEqual({ length: 0 });
+
+        obj = { 0: 1, length: 0 };
+        expect(Array.prototype.sort.call(obj, undefined)).toBe(obj);
+        expect(obj).toEqual({ 0: 1, length: 0 });
+
+        obj = { 0: 3, 1: 2, 2: 1, 3: 0, length: 2 };
+        expect(Array.prototype.sort.call(obj)).toBe(obj);
+        expect(obj).toEqual({ 0: 2, 1: 3, 2: 1, 3: 0, length: 2 });
+
+        obj = { 0: 3, 1: 2, 2: 1, a: "b", hello: "friends!", length: 2 };
+        expect(Array.prototype.sort.call(obj)).toBe(obj);
+        expect(obj).toEqual({ 0: 2, 1: 3, 2: 1, 3: 0, a: "b", hello: "friends!", length: 2 });
+
+        obj = { 0: 2, 1: 3, 2: 1, a: "b", hello: "friends!", length: 2 };
+        expect(
+            Array.prototype.sort.call(obj, (a, b) => {
+                expect(a == 2 || a == 3).toBeTrue();
+                expect(b == 2 || b == 3).toBeTrue();
+                return b - a;
+            })
+        ).toBe(obj);
+        expect(obj).toEqual({ 0: 3, 1: 2, 2: 1, 3: 0, a: "b", hello: "friends!", length: 2 });
+    });
+
+    test("that it handles abrupt completions correctly", () => {
+        class TestError extends Error {
+            constructor() {
+                super();
+                this.name = "TestError";
+            }
+        }
+
+        arr = [1, 2, 3];
+        expect(() =>
+            arr.sort((a, b) => {
+                throw new TestError();
+            })
+        ).toThrow(TestError);
+
+        class DangerousToString {
+            toString() {
+                throw new TestError();
+            }
+        }
+        arr = [new DangerousToString(), new DangerousToString()];
+        expect(() => arr.sort()).toThrow(TestError);
+    });
+
+    test("that it does not use deleteProperty unnecessarily", () => {
+        var obj = new Proxy(
+            { 0: 5, 1: 4, 2: 3, length: 3 },
+            {
+                deleteProperty: function (target, property) {
+                    expect().fail();
+                },
+            }
+        );
+        Array.prototype.sort.call(obj);
+    });
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.splice.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.splice.js
@ -0,0 +1,49 @@
+test("length is 2", () => {
+    expect(Array.prototype.splice).toHaveLength(2);
+});
+
+test("basic functionality", () => {
+    var array = ["hello", "friends", "serenity", 1, 2];
+    var removed = array.splice(3);
+    expect(array).toEqual(["hello", "friends", "serenity"]);
+    expect(removed).toEqual([1, 2]);
+
+    array = ["hello", "friends", "serenity", 1, 2];
+    removed = array.splice(-2);
+    expect(array).toEqual(["hello", "friends", "serenity"]);
+    expect(removed).toEqual([1, 2]);
+
+    array = ["hello", "friends", "serenity", 1, 2];
+    removed = array.splice(-2, 1);
+    expect(array).toEqual(["hello", "friends", "serenity", 2]);
+    expect(removed).toEqual([1]);
+
+    array = ["serenity"];
+    removed = array.splice(0, 0, "hello", "friends");
+    expect(array).toEqual(["hello", "friends", "serenity"]);
+    expect(removed).toEqual([]);
+
+    array = ["goodbye", "friends", "serenity"];
+    removed = array.splice(0, 1, "hello");
+    expect(array).toEqual(["hello", "friends", "serenity"]);
+    expect(removed).toEqual(["goodbye"]);
+
+    array = ["foo", "bar", "baz"];
+    removed = array.splice();
+    expect(array).toEqual(["foo", "bar", "baz"]);
+    expect(removed).toEqual([]);
+
+    removed = array.splice(0, 123);
+    expect(array).toEqual([]);
+    expect(removed).toEqual(["foo", "bar", "baz"]);
+
+    array = ["foo", "bar", "baz"];
+    removed = array.splice(123, 123);
+    expect(array).toEqual(["foo", "bar", "baz"]);
+    expect(removed).toEqual([]);
+
+    array = ["foo", "bar", "baz"];
+    removed = array.splice(-123, 123);
+    expect(array).toEqual([]);
+    expect(removed).toEqual(["foo", "bar", "baz"]);
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.toLocaleString.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.toLocaleString.js
@ -0,0 +1,62 @@
+test("length is 0", () => {
+    expect(Array.prototype.toLocaleString).toHaveLength(0);
+});
+
+describe("normal behavior", () => {
+    test("array with no elements", () => {
+        expect([].toLocaleString()).toBe("");
+    });
+
+    test("array with one element", () => {
+        expect(["foo"].toLocaleString()).toBe("foo");
+    });
+
+    test("array with multiple elements", () => {
+        expect(["foo", "bar", "baz"].toLocaleString()).toBe("foo,bar,baz");
+    });
+
+    test("null and undefined result in empty strings", () => {
+        expect([null].toLocaleString()).toBe("");
+        expect([undefined].toLocaleString()).toBe("");
+        expect([undefined, null].toLocaleString()).toBe(",");
+    });
+
+    test("empty values result in empty strings", () => {
+        expect(new Array(1).toLocaleString()).toBe("");
+        expect(new Array(3).toLocaleString()).toBe(",,");
+        var a = new Array(5);
+        a[2] = "foo";
+        a[4] = "bar";
+        expect(a.toLocaleString()).toBe(",,foo,,bar");
+    });
+
+    test("getter property is included in returned string", () => {
+        var a = ["foo"];
+        Object.defineProperty(a, 1, {
+            get() {
+                return "bar";
+            },
+        });
+        expect(a.toLocaleString()).toBe("foo,bar");
+    });
+
+    test("array with elements that have a custom toString() function", () => {
+        var toStringCalled = 0;
+        var o = {
+            toString() {
+                toStringCalled++;
+                return "o";
+            },
+        };
+        expect([o, undefined, o, null, o].toLocaleString()).toBe("o,,o,,o");
+        expect(toStringCalled).toBe(3);
+    });
+
+    test("array with circular references", () => {
+        const a = ["foo", [], [1, 2, []], ["bar"]];
+        a[1] = a;
+        a[2][2] = a;
+        // [ "foo", <circular>, [ 1, 2, <circular> ], [ "bar" ] ]
+        expect(a.toLocaleString()).toBe("foo,,1,2,,bar");
+    });
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.toString.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.toString.js
@ -0,0 +1,66 @@
+test("length is 0", () => {
+    expect(Array.prototype.toString).toHaveLength(0);
+});
+
+describe("normal behavior", () => {
+    test("array with no elements", () => {
+        expect([].toString()).toBe("");
+    });
+
+    test("array with one element", () => {
+        expect([1].toString()).toBe("1");
+    });
+
+    test("array with multiple elements", () => {
+        expect([1, 2, 3].toString()).toBe("1,2,3");
+    });
+
+    test("string and array concatenation", () => {
+        expect("rgb(" + [10, 11, 12] + ")").toBe("rgb(10,11,12)");
+    });
+
+    test("null and undefined result in empty strings", () => {
+        expect([null].toString()).toBe("");
+        expect([undefined].toString()).toBe("");
+        expect([undefined, null].toString()).toBe(",");
+    });
+
+    test("empty values result in empty strings", () => {
+        expect(new Array(1).toString()).toBe("");
+        expect(new Array(3).toString()).toBe(",,");
+        var a = new Array(5);
+        a[2] = "foo";
+        a[4] = "bar";
+        expect(a.toString()).toBe(",,foo,,bar");
+    });
+
+    test("getter property is included in returned string", () => {
+        var a = [1, 2, 3];
+        Object.defineProperty(a, 3, {
+            get() {
+                return 10;
+            },
+        });
+        expect(a.toString()).toBe("1,2,3,10");
+    });
+
+    test("array with elements that have a custom toString() function", () => {
+        var toStringCalled = 0;
+        var o = {
+            toString() {
+                toStringCalled++;
+                return "o";
+            },
+        };
+        expect([o, undefined, o, null, o].toString()).toBe("o,,o,,o");
+        expect(toStringCalled).toBe(3);
+    });
+
+    test("array with circular references", () => {
+        const a = ["foo", [], [1, 2, []], ["bar"]];
+        a[1] = a;
+        a[2][2] = a;
+        // [ "foo", <circular>, [ 1, 2, <circular> ], [ "bar" ] ]
+        expect(a.toString()).toBe("foo,,1,2,,bar");
+    });
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.unshift.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.unshift.js
@ -0,0 +1,23 @@
+test("length is 1", () => {
+    expect(Array.prototype.unshift).toHaveLength(1);
+});
+
+describe("normal behavior", () => {
+    test("no argument", () => {
+        var a = ["hello"];
+        expect(a.unshift()).toBe(1);
+        expect(a).toEqual(["hello"]);
+    });
+
+    test("single argument", () => {
+        var a = ["hello"];
+        expect(a.unshift("friends")).toBe(2);
+        expect(a).toEqual(["friends", "hello"]);
+    });
+
+    test("multiple arguments", () => {
+        var a = ["friends", "hello"];
+        expect(a.unshift(1, 2, 3)).toBe(5);
+        expect(a).toEqual([1, 2, 3, "friends", "hello"]);
+    });
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.values.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.values.js
@ -0,0 +1,44 @@
+test("length", () => {
+    expect(Array.prototype.values.length).toBe(0);
+});
+
+test("basic functionality", () => {
+    const a = [1, 2, 3];
+    const it = a.values();
+    expect(it.next()).toEqual({ value: 1, done: false });
+    expect(it.next()).toEqual({ value: 2, done: false });
+    expect(it.next()).toEqual({ value: 3, done: false });
+    expect(it.next()).toEqual({ value: undefined, done: true });
+    expect(it.next()).toEqual({ value: undefined, done: true });
+    expect(it.next()).toEqual({ value: undefined, done: true });
+});
+
+test("works when applied to non-object", () => {
+    [true, false, 9, 2n, Symbol()].forEach(primitive => {
+        const it = [].values.call(primitive);
+        expect(it.next()).toEqual({ value: undefined, done: true });
+        expect(it.next()).toEqual({ value: undefined, done: true });
+        expect(it.next()).toEqual({ value: undefined, done: true });
+    });
+});
+
+test("item added to array before exhaustion is accessible", () => {
+    const a = [1, 2];
+    const it = a.values();
+    expect(it.next()).toEqual({ value: 1, done: false });
+    expect(it.next()).toEqual({ value: 2, done: false });
+    a.push(3);
+    expect(it.next()).toEqual({ value: 3, done: false });
+    expect(it.next()).toEqual({ value: undefined, done: true });
+    expect(it.next()).toEqual({ value: undefined, done: true });
+});
+
+test("item added to array after exhaustion is inaccessible", () => {
+    const a = [1, 2];
+    const it = a.values();
+    expect(it.next()).toEqual({ value: 1, done: false });
+    expect(it.next()).toEqual({ value: 2, done: false });
+    expect(it.next()).toEqual({ value: undefined, done: true });
+    a.push(3);
+    expect(it.next()).toEqual({ value: undefined, done: true });
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Array/array-basic.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Array/array-basic.js
@ -0,0 +1,57 @@
+test("basic functionality", () => {
+    var a = [1, 2, 3];
+
+    expect(typeof a).toBe("object");
+    expect(a).toHaveLength(3);
+    expect(a[0]).toBe(1);
+    expect(a[1]).toBe(2);
+    expect(a[2]).toBe(3);
+
+    a[1] = 5;
+    expect(a[1]).toBe(5);
+    expect(a).toHaveLength(3);
+
+    a.push(7);
+    expect(a[3]).toBe(7);
+    expect(a).toHaveLength(4);
+
+    a = [,];
+    expect(a).toHaveLength(1);
+    expect(a.toString()).toBe("");
+    expect(a[0]).toBeUndefined();
+
+    a = [, , , ,];
+    expect(a).toHaveLength(4);
+    expect(a.toString()).toBe(",,,");
+    expect(a[0]).toBeUndefined();
+    expect(a[1]).toBeUndefined();
+    expect(a[2]).toBeUndefined();
+    expect(a[3]).toBeUndefined();
+
+    a = [1, , 2, , , 3];
+    expect(a).toHaveLength(6);
+    expect(a.toString()).toBe("1,,2,,,3");
+    expect(a[0]).toBe(1);
+    expect(a[1]).toBeUndefined();
+    expect(a[2]).toBe(2);
+    expect(a[3]).toBeUndefined();
+    expect(a[4]).toBeUndefined();
+    expect(a[5]).toBe(3);
+
+    a = [1, , 2, , , 3];
+    Object.defineProperty(a, 1, {
+        get() {
+            return this.getterSetterValue;
+        },
+        set(value) {
+            this.getterSetterValue = value;
+        },
+    });
+    expect(a).toHaveLength(6);
+    expect(a.toString()).toBe("1,,2,,,3");
+    expect(a.getterSetterValue).toBeUndefined();
+    a[1] = 20;
+    expect(a).toHaveLength(6);
+    expect(a.toString()).toBe("1,20,2,,,3");
+    expect(a.getterSetterValue).toBe(20);
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Array/array-length-setter.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Array/array-length-setter.js
@ -0,0 +1,37 @@
+describe("errors", () => {
+    test("invalid array length value", () => {
+        var a = [1, 2, 3];
+        [undefined, "foo", -1, Infinity, -Infinity, NaN].forEach(value => {
+            expect(() => {
+                a.length = value;
+            }).toThrowWithMessage(RangeError, "Invalid array length");
+            expect(a).toHaveLength(3);
+        });
+    });
+});
+
+describe("normal behavior", () => {
+    test("extend array by setting length", () => {
+        var a = [1, 2, 3];
+        a.length = 5;
+        expect(a).toEqual([1, 2, 3, undefined, undefined]);
+    });
+
+    test("truncate array by setting length", () => {
+        var a = [1, 2, 3];
+        a.length = 2;
+        expect(a).toEqual([1, 2]);
+        a.length = 0;
+        expect(a).toEqual([]);
+    });
+
+    test("length value is coerced to number if possible", () => {
+        var a = [1, 2, 3];
+        a.length = "42";
+        expect(a).toHaveLength(42);
+        a.length = [];
+        expect(a).toHaveLength(0);
+        a.length = true;
+        expect(a).toHaveLength(1);
+    });
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Array/array-shrink-during-find-crash.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Array/array-shrink-during-find-crash.js
@ -0,0 +1,19 @@
+test("Issue #1992, shrinking array during find() iteration", () => {
+    var a, callbackCalled;
+
+    callbackCalled = 0;
+    a = [1, 2, 3, 4, 5];
+    a.find(() => {
+        callbackCalled++;
+        a.pop();
+    });
+    expect(callbackCalled).toBe(5);
+
+    callbackCalled = 0;
+    a = [1, 2, 3, 4, 5];
+    a.findIndex(() => {
+        callbackCalled++;
+        a.pop();
+    });
+    expect(callbackCalled).toBe(5);
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Array/array-simple-and-generic-storage-initialization.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Array/array-simple-and-generic-storage-initialization.js
@ -0,0 +1,15 @@
+describe("Issue #3382", () => {
+    test("Creating an array with simple storage (<= 200 initial elements)", () => {
+        var a = Array(200);
+        expect(a).toHaveLength(200);
+        expect(a.push("foo")).toBe(201);
+        expect(a).toHaveLength(201);
+    });
+
+    test("Creating an array with generic storage (> 200 initial elements)", () => {
+        var a = Array(201);
+        expect(a).toHaveLength(201);
+        expect(a.push("foo")).toBe(202);
+        expect(a).toHaveLength(202);
+    });
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Array/array-spread.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Array/array-spread.js
@ -0,0 +1,25 @@
+describe("errors", () => {
+    test("cannot spread number in array", () => {
+        expect(() => {
+            [...1];
+        }).toThrowWithMessage(TypeError, "1 is not iterable");
+    });
+
+    test("cannot spread object in array", () => {
+        expect(() => {
+            [...{}];
+        }).toThrowWithMessage(TypeError, "[object Object] is not iterable");
+    });
+});
+
+test("basic functionality", () => {
+    expect([1, ...[2, 3], 4]).toEqual([1, 2, 3, 4]);
+
+    let a = [2, 3];
+    expect([1, ...a, 4]).toEqual([1, 2, 3, 4]);
+
+    let obj = { a: [2, 3] };
+    expect([1, ...obj.a, 4]).toEqual([1, 2, 3, 4]);
+
+    expect([...[], ...[...[1, 2, 3]], 4]).toEqual([1, 2, 3, 4]);
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/ArrayBuffer/ArrayBuffer.isView.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/ArrayBuffer/ArrayBuffer.isView.js
@ -0,0 +1,27 @@
+// Update when more typed arrays get added
+const TYPED_ARRAYS = [
+    Uint8Array,
+    Uint16Array,
+    Uint32Array,
+    Int8Array,
+    Int16Array,
+    Int32Array,
+    Float32Array,
+    Float64Array,
+];
+
+test("basic functionality", () => {
+    expect(ArrayBuffer.isView).toHaveLength(1);
+
+    expect(ArrayBuffer.isView()).toBeFalse();
+    expect(ArrayBuffer.isView(null)).toBeFalse();
+    expect(ArrayBuffer.isView(undefined)).toBeFalse();
+    expect(ArrayBuffer.isView([])).toBeFalse();
+    expect(ArrayBuffer.isView({})).toBeFalse();
+    expect(ArrayBuffer.isView(123)).toBeFalse();
+    expect(ArrayBuffer.isView("foo")).toBeFalse();
+    expect(ArrayBuffer.isView(new ArrayBuffer())).toBeFalse();
+    TYPED_ARRAYS.forEach(T => {
+        expect(ArrayBuffer.isView(new T())).toBeTrue();
+    });
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/ArrayBuffer/ArrayBuffer.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/ArrayBuffer/ArrayBuffer.js
@ -0,0 +1,13 @@
+test("basic functionality", () => {
+    expect(ArrayBuffer).toHaveLength(1);
+    expect(ArrayBuffer.name).toBe("ArrayBuffer");
+    expect(ArrayBuffer.prototype.constructor).toBe(ArrayBuffer);
+    expect(new ArrayBuffer()).toBeInstanceOf(ArrayBuffer);
+    expect(typeof new ArrayBuffer()).toBe("object");
+});
+
+test("ArrayBuffer constructor must be invoked with 'new'", () => {
+    expect(() => {
+        ArrayBuffer();
+    }).toThrowWithMessage(TypeError, "ArrayBuffer constructor must be called with 'new'");
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/ArrayBuffer/ArrayBuffer.prototype.byteLength.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/ArrayBuffer/ArrayBuffer.prototype.byteLength.js
@ -0,0 +1,6 @@
+test("basic functionality", () => {
+    expect(new ArrayBuffer().byteLength).toBe(0);
+    expect(new ArrayBuffer(1).byteLength).toBe(1);
+    expect(new ArrayBuffer(64).byteLength).toBe(64);
+    expect(new ArrayBuffer(123).byteLength).toBe(123);
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/BigInt/BigInt.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/BigInt/BigInt.js
@ -0,0 +1,68 @@
+describe("correct behavior", () => {
+    test("basic functionality", () => {
+        expect(BigInt).toHaveLength(1);
+        expect(BigInt.name).toBe("BigInt");
+    });
+
+    test("constructor with numbers", () => {
+        expect(BigInt(0)).toBe(0n);
+        expect(BigInt(1)).toBe(1n);
+        expect(BigInt(+1)).toBe(1n);
+        expect(BigInt(-1)).toBe(-1n);
+        expect(BigInt(123n)).toBe(123n);
+    });
+
+    test("constructor with strings", () => {
+        expect(BigInt("")).toBe(0n);
+        expect(BigInt("0")).toBe(0n);
+        expect(BigInt("1")).toBe(1n);
+        expect(BigInt("+1")).toBe(1n);
+        expect(BigInt("-1")).toBe(-1n);
+        expect(BigInt("-1")).toBe(-1n);
+        expect(BigInt("42")).toBe(42n);
+        expect(BigInt("  \n  00100  \n  ")).toBe(100n);
+        expect(BigInt("3323214327642987348732109829832143298746432437532197321")).toBe(
+            3323214327642987348732109829832143298746432437532197321n
+        );
+    });
+
+    test("constructor with objects", () => {
+        expect(BigInt([])).toBe(0n);
+    });
+});
+
+describe("errors", () => {
+    test('cannot be constructed with "new"', () => {
+        expect(() => {
+            new BigInt();
+        }).toThrowWithMessage(TypeError, "BigInt is not a constructor");
+    });
+
+    test("invalid arguments", () => {
+        expect(() => {
+            BigInt(null);
+        }).toThrowWithMessage(TypeError, "Cannot convert null to BigInt");
+
+        expect(() => {
+            BigInt(undefined);
+        }).toThrowWithMessage(TypeError, "Cannot convert undefined to BigInt");
+
+        expect(() => {
+            BigInt(Symbol());
+        }).toThrowWithMessage(TypeError, "Cannot convert symbol to BigInt");
+
+        ["foo", "123n", "1+1", {}, function () {}].forEach(value => {
+            expect(() => {
+                BigInt(value);
+            }).toThrowWithMessage(SyntaxError, `Invalid value for BigInt: ${value}`);
+        });
+    });
+
+    test("invalid numeric arguments", () => {
+        [1.23, Infinity, -Infinity, NaN].forEach(value => {
+            expect(() => {
+                BigInt(value);
+            }).toThrowWithMessage(RangeError, "BigInt argument must be an integer");
+        });
+    });
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/BigInt/BigInt.prototype.@@toStringTag.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/BigInt/BigInt.prototype.@@toStringTag.js
@ -0,0 +1,3 @@
+test("basic functionality", () => {
+    expect(BigInt.prototype[Symbol.toStringTag]).toBe("BigInt");
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/BigInt/BigInt.prototype.toLocaleString.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/BigInt/BigInt.prototype.toLocaleString.js
@ -0,0 +1,10 @@
+test("basic functionality", () => {
+    expect(BigInt.prototype.toLocaleString).toHaveLength(0);
+    expect(BigInt(123).toLocaleString()).toBe("123");
+});
+
+test("calling with non-BigInt |this|", () => {
+    expect(() => {
+        BigInt.prototype.toLocaleString.call("foo");
+    }).toThrowWithMessage(TypeError, "Not a BigInt object");
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/BigInt/BigInt.prototype.toString.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/BigInt/BigInt.prototype.toString.js
@ -0,0 +1,10 @@
+test("basic functionality", () => {
+    expect(BigInt.prototype.toString).toHaveLength(0);
+    expect(BigInt(123).toString()).toBe("123");
+});
+
+test("calling with non-BigInt |this|", () => {
+    expect(() => {
+        BigInt.prototype.toString.call("foo");
+    }).toThrowWithMessage(TypeError, "Not a BigInt object");
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/BigInt/BigInt.prototype.valueOf.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/BigInt/BigInt.prototype.valueOf.js
@ -0,0 +1,12 @@
+test("basic functionality", () => {
+    expect(BigInt.prototype.valueOf).toHaveLength(0);
+    expect(typeof BigInt(123).valueOf()).toBe("bigint");
+    // FIXME: Uncomment once we support Object() with argument
+    // expect(typeof Object(123n).valueOf()).toBe("bigint");
+});
+
+test("calling with non-BigInt |this|", () => {
+    expect(() => {
+        BigInt.prototype.valueOf.call("foo");
+    }).toThrowWithMessage(TypeError, "Not a BigInt object");
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/BigInt/bigint-basic.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/BigInt/bigint-basic.js
@ -0,0 +1,80 @@
+describe("correct behavior", () => {
+    test("typeof bigint", () => {
+        expect(typeof 1n).toBe("bigint");
+    });
+
+    test("bigint string coercion", () => {
+        expect("" + 123n).toBe("123");
+    });
+
+    test("arithmetic operators", () => {
+        let bigint = 123n;
+        expect(-bigint).toBe(-123n);
+
+        expect(12n + 34n).toBe(46n);
+        expect(12n - 34n).toBe(-22n);
+        expect(8n * 12n).toBe(96n);
+        expect(123n / 10n).toBe(12n);
+        expect(2n ** 3n).toBe(8n);
+        expect(5n % 3n).toBe(2n);
+        expect(
+            45977665298704210987n +
+                (714320987142450987412098743217984576n / 4598741987421098765327980n) * 987498743n
+        ).toBe(199365500239020623962n);
+    });
+
+    test("bitwise operators", () => {
+        expect(12n & 5n).toBe(4n);
+        expect(1n | 2n).toBe(3n);
+        expect(5n ^ 3n).toBe(6n);
+        expect(~1n).toBe(-2n);
+    });
+
+    test("increment operators", () => {
+        let bigint = 1n;
+        expect(bigint++).toBe(1n);
+        expect(bigint).toBe(2n);
+        expect(bigint--).toBe(2n);
+        expect(bigint).toBe(1n);
+        expect(++bigint).toBe(2n);
+        expect(bigint).toBe(2n);
+        expect(--bigint).toBe(1n);
+        expect(bigint).toBe(1n);
+    });
+
+    test("weak equality operators", () => {
+        expect(1n == 1n).toBeTrue();
+        expect(1n == 1).toBeTrue();
+        expect(1 == 1n).toBeTrue();
+        expect(1n == 1.23).toBeFalse();
+        expect(1.23 == 1n).toBeFalse();
+
+        expect(1n != 1n).toBeFalse();
+        expect(1n != 1).toBeFalse();
+        expect(1 != 1n).toBeFalse();
+        expect(1n != 1.23).toBeTrue();
+        expect(1.23 != 1n).toBeTrue();
+    });
+
+    test("strong equality operators", () => {
+        expect(1n === 1n).toBeTrue();
+        expect(1n === 1).toBeFalse();
+        expect(1 === 1n).toBeFalse();
+        expect(1n === 1.23).toBeFalse();
+        expect(1.23 === 1n).toBeFalse();
+
+        expect(1n !== 1n).toBeFalse();
+        expect(1n !== 1).toBeTrue();
+        expect(1 !== 1n).toBeTrue();
+        expect(1n !== 1.23).toBeTrue();
+        expect(1.23 !== 1n).toBeTrue();
+    });
+});
+
+describe("errors", () => {
+    test("conversion to number", () => {
+        expect(() => {
+            +123n;
+        }).toThrowWithMessage(TypeError, "Cannot convert BigInt to number");
+    });
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/BigInt/bigint-minus.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/BigInt/bigint-minus.js
@ -0,0 +1,8 @@
+describe("minus behavior", () => {
+    test("the basics", () => {
+        expect(3n - 4n).toBe(-1n);
+        expect(3n - -4n).toBe(7n);
+        expect(-3n - -4n).toBe(-1n);
+        expect(-3n - 4n).toBe(-7n);
+    });
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/BigInt/bigint-number-mix-errors.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/BigInt/bigint-number-mix-errors.js
@ -0,0 +1,31 @@
+const doTest = (operatorName, executeOperation) => {
+    [1, null, undefined].forEach(value => {
+        const messageSuffix = operatorName === "unsigned right-shift" ? "" : " and other type";
+
+        expect(() => {
+            executeOperation(1n, value);
+        }).toThrowWithMessage(
+            TypeError,
+            `Cannot use ${operatorName} operator with BigInt${messageSuffix}`
+        );
+    });
+};
+
+[
+    ["addition", (a, b) => a + b],
+    ["subtraction", (a, b) => a - b],
+    ["multiplication", (a, b) => a * b],
+    ["division", (a, b) => a / b],
+    ["modulo", (a, b) => a % b],
+    ["exponentiation", (a, b) => a ** b],
+    ["bitwise OR", (a, b) => a | b],
+    ["bitwise AND", (a, b) => a & b],
+    ["bitwise XOR", (a, b) => a ^ b],
+    ["left-shift", (a, b) => a << b],
+    ["right-shift", (a, b) => a >> b],
+    ["unsigned right-shift", (a, b) => a >>> b],
+].forEach(testCase => {
+    test(`using ${testCase[0]} operator with BigInt and other type`, () => {
+        doTest(testCase[0], testCase[1]);
+    });
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Boolean/Boolean.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Boolean/Boolean.js
@ -0,0 +1,32 @@
+test("constructor properties", () => {
+    expect(Boolean).toHaveLength(1);
+    expect(Boolean.name).toBe("Boolean");
+});
+
+test("typeof", () => {
+    expect(typeof new Boolean()).toBe("object");
+    expect(typeof Boolean()).toBe("boolean");
+    expect(typeof Boolean(true)).toBe("boolean");
+});
+
+test("basic functionality", () => {
+    var foo = new Boolean(true);
+    var bar = new Boolean(true);
+
+    expect(foo).not.toBe(bar);
+    expect(foo.valueOf()).toBe(bar.valueOf());
+
+    expect(Boolean()).toBeFalse();
+    expect(Boolean(false)).toBeFalse();
+    expect(Boolean(null)).toBeFalse();
+    expect(Boolean(undefined)).toBeFalse();
+    expect(Boolean(NaN)).toBeFalse();
+    expect(Boolean("")).toBeFalse();
+    expect(Boolean(0.0)).toBeFalse();
+    expect(Boolean(-0.0)).toBeFalse();
+    expect(Boolean(true)).toBeTrue();
+    expect(Boolean("0")).toBeTrue();
+    expect(Boolean({})).toBeTrue();
+    expect(Boolean([])).toBeTrue();
+    expect(Boolean(1)).toBeTrue();
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Boolean/Boolean.prototype.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Boolean/Boolean.prototype.js
@ -0,0 +1,5 @@
+test("basic functionality", () => {
+    expect(typeof Boolean.prototype).toBe("object");
+    expect(Boolean.prototype.valueOf()).toBeFalse();
+    expect(Boolean.prototype).not.toHaveProperty("length");
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Boolean/Boolean.prototype.toString.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Boolean/Boolean.prototype.toString.js
@ -0,0 +1,17 @@
+test("basic functionality", () => {
+    var foo = true;
+    expect(foo.toString()).toBe("true");
+    expect(true.toString()).toBe("true");
+
+    expect(Boolean.prototype.toString.call(true)).toBe("true");
+    expect(Boolean.prototype.toString.call(false)).toBe("false");
+
+    expect(new Boolean(true).toString()).toBe("true");
+    expect(new Boolean(false).toString()).toBe("false");
+});
+
+test("errors on non-boolean |this|", () => {
+    expect(() => {
+        Boolean.prototype.toString.call("foo");
+    }).toThrowWithMessage(TypeError, "Not a Boolean object");
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Boolean/Boolean.prototype.valueOf.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Boolean/Boolean.prototype.valueOf.js
@ -0,0 +1,16 @@
+test("basic functionality", () => {
+    var foo = true;
+    expect(foo.valueOf()).toBeTrue();
+    expect(true.valueOf()).toBeTrue();
+
+    expect(Boolean.prototype.valueOf.call(true)).toBeTrue();
+    expect(Boolean.prototype.valueOf.call(false)).toBeFalse();
+
+    expect(new Boolean().valueOf()).toBeFalse();
+});
+
+test("errors on non-boolean |this|", () => {
+    expect(() => {
+        Boolean.prototype.valueOf.call("foo");
+    }).toThrowWithMessage(TypeError, "Not a Boolean object");
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Date/Date.UTC.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Date/Date.UTC.js
@ -0,0 +1,51 @@
+test("basic functionality", () => {
+    expect(Date.UTC(2020)).toBe(1577836800000);
+    expect(Date.UTC(2000, 10)).toBe(973036800000);
+    expect(Date.UTC(1980, 5, 30)).toBe(331171200000);
+    expect(Date.UTC(1980, 5, 30, 13)).toBe(331218000000);
+    expect(Date.UTC(1970, 5, 30, 13, 30)).toBe(15600600000);
+    expect(Date.UTC(1970, 0, 1, 0, 0, 59)).toBe(59000);
+    expect(Date.UTC(1970, 0, 1, 0, 0, 0, 999)).toBe(999);
+
+    expect(Date.UTC(1969, 11, 31, 23, 59, 59, 817)).toBe(-183);
+
+    expect(Date.UTC(1799, 0)).toBe(-5396198400000);
+    expect(Date.UTC(1800, 0)).toBe(-5364662400000);
+    expect(Date.UTC(1801, 0)).toBe(-5333126400000);
+    expect(Date.UTC(1802, 0)).toBe(-5301590400000);
+    expect(Date.UTC(1803, 0)).toBe(-5270054400000);
+    expect(Date.UTC(1804, 0)).toBe(-5238518400000);
+
+    expect(Date.UTC(1999, 0)).toBe(915148800000);
+    expect(Date.UTC(2000, 0)).toBe(946684800000);
+    expect(Date.UTC(2001, 0)).toBe(978307200000);
+    expect(Date.UTC(2002, 0)).toBe(1009843200000);
+    expect(Date.UTC(2003, 0)).toBe(1041379200000);
+    expect(Date.UTC(2004, 0)).toBe(1072915200000);
+
+    expect(Date.UTC(20000, 0)).toBe(568971820800000);
+});
+
+test("leap year", () => {
+    expect(Date.UTC(2020, 2, 1)).toBe(1583020800000);
+});
+
+test("out of range", () => {
+    expect(Date.UTC(2020, -20)).toBe(1525132800000);
+    expect(Date.UTC(2020, 20)).toBe(1630454400000);
+
+    expect(Date.UTC(2020, 1, -10)).toBe(1579564800000);
+    expect(Date.UTC(2020, 1, 40)).toBe(1583884800000);
+
+    expect(Date.UTC(2020, 1, 15, -50)).toBe(1581544800000);
+    expect(Date.UTC(2020, 1, 15, 50)).toBe(1581904800000);
+
+    expect(Date.UTC(2020, 1, 15, 12, -123)).toBe(1581760620000);
+    expect(Date.UTC(2020, 1, 15, 12, 123)).toBe(1581775380000);
+
+    expect(Date.UTC(2020, 1, 15, 12, 30, -123)).toBe(1581769677000);
+    expect(Date.UTC(2020, 1, 15, 12, 30, 123)).toBe(1581769923000);
+
+    expect(Date.UTC(2020, 1, 15, 12, 30, 30, -2345)).toBe(1581769827655);
+    expect(Date.UTC(2020, 1, 15, 12, 30, 30, 2345)).toBe(1581769832345);
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Date/Date.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Date/Date.js
@ -0,0 +1,77 @@
+test("basic functionality", () => {
+    expect(Date).toHaveLength(7);
+    expect(Date.name === "Date");
+    expect(Date.prototype).not.toHaveProperty("length");
+});
+
+test("string constructor", () => {
+    // The string constructor is the same as calling the timestamp constructor with the result of Date.parse(arguments).
+    // Since that has exhaustive tests in Date.parse.js, just do some light smoke testing here.
+    expect(new Date("2017-09-07T21:08:59.001Z").toISOString()).toBe("2017-09-07T21:08:59.001Z");
+});
+
+test("timestamp constructor", () => {
+    // The timestamp constructor takes a timestamp in milliseconds since the start of the epoch, in UTC.
+
+    // 50 days and 1234 milliseconds after the start of the epoch.
+    // Most Date methods return values in local time, but since timezone offsets are less than 17 days,
+    // these checks will pass in all timezones.
+    let timestamp = 50 * 24 * 60 * 60 * 1000 + 1234;
+
+    let date = new Date(timestamp);
+    expect(date.getTime()).toBe(timestamp); // getTime() returns the timestamp in UTC.
+    expect(date.getMilliseconds()).toBe(234);
+    expect(date.getSeconds()).toBe(1);
+    expect(date.getFullYear()).toBe(1970);
+    expect(date.getMonth()).toBe(1); // Feb
+});
+
+test("tuple constructor", () => {
+    // The tuple constructor takes a date in local time.
+    expect(new Date(2019, 11).getFullYear()).toBe(2019);
+    expect(new Date(2019, 11).getMonth()).toBe(11);
+    expect(new Date(2019, 11).getDate()).toBe(1); // getDay() returns day of week, getDate() returns day in month
+    expect(new Date(2019, 11).getHours()).toBe(0);
+    expect(new Date(2019, 11).getMinutes()).toBe(0);
+    expect(new Date(2019, 11).getSeconds()).toBe(0);
+    expect(new Date(2019, 11).getMilliseconds()).toBe(0);
+    expect(new Date(2019, 11).getDay()).toBe(0);
+
+    let date = new Date(2019, 11, 15, 9, 16, 14, 123); // Note: Month is 0-based.
+    expect(date.getFullYear()).toBe(2019);
+    expect(date.getMonth()).toBe(11);
+    expect(date.getDate()).toBe(15);
+    expect(date.getHours()).toBe(9);
+    expect(date.getMinutes()).toBe(16);
+    expect(date.getSeconds()).toBe(14);
+    expect(date.getMilliseconds()).toBe(123);
+    expect(date.getDay()).toBe(0);
+
+    // getTime() returns a time stamp in UTC, but we can at least check it's in the right interval, which will be true independent of the local timezone if the range is big enough.
+    let timestamp_lower_bound = 1575072000000; // 2019-12-01T00:00:00Z
+    let timestamp_upper_bound = 1577750400000; // 2019-12-31T00:00:00Z
+    expect(date.getTime()).toBeGreaterThan(timestamp_lower_bound);
+    expect(date.getTime()).toBeLessThan(timestamp_upper_bound);
+});
+
+test("tuple constructor overflow", () => {
+    let date = new Date(2019, 13, 33, 30, 70, 80, 2345);
+    expect(date.getFullYear()).toBe(2020);
+    expect(date.getMonth()).toBe(2);
+    expect(date.getDate()).toBe(5);
+    expect(date.getHours()).toBe(7);
+    expect(date.getMinutes()).toBe(11);
+    expect(date.getSeconds()).toBe(22);
+    expect(date.getMilliseconds()).toBe(345);
+    expect(date.getDay()).toBe(4);
+
+    let date = new Date(2019, -13, -33, -30, -70, -80, -2345);
+    expect(date.getFullYear()).toBe(2017);
+    expect(date.getMonth()).toBe(9);
+    expect(date.getDate()).toBe(26);
+    expect(date.getHours()).toBe(16);
+    expect(date.getMinutes()).toBe(48);
+    expect(date.getSeconds()).toBe(37);
+    expect(date.getMilliseconds()).toBe(655);
+    expect(date.getDay()).toBe(4);
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Date/Date.now.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Date/Date.now.js
@ -0,0 +1,10 @@
+test("basic functionality", () => {
+    var last = 0;
+    for (var i = 0; i < 100; ++i) {
+        var now = Date.now();
+        expect(now).not.toBeNaN();
+        expect(now).toBeGreaterThan(1580000000000);
+        expect(now).toBeGreaterThanOrEqual(last);
+        last = now;
+    }
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Date/Date.parse.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Date/Date.parse.js
@ -0,0 +1,32 @@
+test("basic functionality", () => {
+    expect(Date.parse("2020")).toBe(1577836800000);
+    expect(Date.parse("2000-11")).toBe(973036800000);
+    expect(Date.parse("1980-06-30")).toBe(331171200000);
+    expect(Date.parse("1970-06-30T13:30Z")).toBe(15600600000);
+    expect(Date.parse("1970-01-01T00:00:59Z")).toBe(59000);
+    expect(Date.parse("1970-01-01T00:00:00.999Z")).toBe(999);
+    expect(Date.parse("2020T13:14+15:16")).toBe(1577829480000);
+    expect(Date.parse("2020T13:14-15:16")).toBe(1577939400000);
+    expect(Date.parse("2020T23:59Z")).toBe(1577923140000);
+
+    expect(Date.parse("+020000")).toBe(568971820800000);
+    expect(Date.parse("+020000-01")).toBe(568971820800000);
+    expect(Date.parse("+020000-01T00:00:00.000Z")).toBe(568971820800000);
+
+    expect(Date.parse(2020)).toBe(1577836800000);
+
+    expect(Date.parse("+1980")).toBe(NaN);
+    expect(Date.parse("1980-")).toBe(NaN);
+    expect(Date.parse("1980-05-")).toBe(NaN);
+    expect(Date.parse("1980-05-00T")).toBe(NaN);
+    expect(Date.parse("1980-05-00T15:15:")).toBe(NaN);
+    expect(Date.parse("1980-05-00T15:15:15.")).toBe(NaN);
+    expect(Date.parse("1980-5-30")).toBe(NaN);
+    expect(Date.parse("1980-05-30T13")).toBe(NaN);
+    expect(Date.parse("1980-05-30T13:4")).toBe(NaN);
+    expect(Date.parse("1980-05-30T13:40+")).toBe(NaN);
+    expect(Date.parse("1980-05-30T13:40+1")).toBe(NaN);
+    expect(Date.parse("1980-05-30T13:40+1:10")).toBe(NaN);
+    expect(Date.parse("1970-06-30T13:30Zoo")).toBe(NaN);
+    expect(Date.parse("2020T13:30.40:")).toBe(NaN);
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Date/Date.prototype.getDate.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Date/Date.prototype.getDate.js
@ -0,0 +1,7 @@
+test("basic functionality", () => {
+    let d = new Date();
+    expect(d.getDate()).toBe(d.getDate());
+    expect(d.getDate()).not.toBeNaN();
+    expect(d.getDate()).toBeGreaterThanOrEqual(1);
+    expect(d.getDate()).toBeLessThanOrEqual(31);
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Date/Date.prototype.getDay.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Date/Date.prototype.getDay.js
@ -0,0 +1,7 @@
+test("basic functionality", () => {
+    var d = new Date();
+    expect(d.getDay()).toBe(d.getDay());
+    expect(d.getDay()).not.toBeNaN();
+    expect(d.getDay()).toBeGreaterThanOrEqual(0);
+    expect(d.getDay()).toBeLessThanOrEqual(6);
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Date/Date.prototype.getFullYear.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Date/Date.prototype.getFullYear.js
@ -0,0 +1,7 @@
+test("basic functionality", () => {
+    var d = new Date();
+    expect(d.getFullYear()).toBe(d.getFullYear());
+    expect(d.getFullYear()).not.toBeNaN();
+    expect(d.getFullYear()).toBe(d.getFullYear());
+    expect(d.getFullYear()).toBeGreaterThanOrEqual(2020);
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Date/Date.prototype.getHours.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Date/Date.prototype.getHours.js
@ -0,0 +1,7 @@
+test("basic functionality", () => {
+    var d = new Date();
+    expect(d.getHours()).toBe(d.getHours());
+    expect(d.getHours()).not.toBeNaN();
+    expect(d.getHours()).toBeGreaterThanOrEqual(0);
+    expect(d.getHours()).toBeLessThanOrEqual(23);
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Date/Date.prototype.getMilliseconds.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Date/Date.prototype.getMilliseconds.js
@ -0,0 +1,7 @@
+test("basic functionality", () => {
+    var d = new Date();
+    expect(d.getMilliseconds()).toBe(d.getMilliseconds());
+    expect(d.getMilliseconds()).not.toBeNaN();
+    expect(d.getMilliseconds()).toBeGreaterThanOrEqual(0);
+    expect(d.getMilliseconds()).toBeLessThanOrEqual(999);
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Date/Date.prototype.getMinutes.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Date/Date.prototype.getMinutes.js
@ -0,0 +1,7 @@
+test("basic functionality", () => {
+    var d = new Date();
+    expect(d.getMinutes()).toBe(d.getMinutes());
+    expect(d.getMinutes()).not.toBeNaN();
+    expect(d.getMinutes()).toBeGreaterThanOrEqual(0);
+    expect(d.getMinutes()).toBeLessThanOrEqual(59);
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Date/Date.prototype.getMonth.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Date/Date.prototype.getMonth.js
@ -0,0 +1,7 @@
+test("basic functionality", () => {
+    var d = new Date();
+    expect(d.getMonth()).toBe(d.getMonth());
+    expect(d.getMonth()).not.toBeNaN();
+    expect(d.getMonth()).toBeGreaterThanOrEqual(0);
+    expect(d.getMonth()).toBeLessThanOrEqual(11);
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Date/Date.prototype.getSeconds.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Date/Date.prototype.getSeconds.js
@ -0,0 +1,7 @@
+test("basic functionality", () => {
+    var d = new Date();
+    expect(d.getSeconds()).toBe(d.getSeconds());
+    expect(d.getSeconds()).not.toBeNaN();
+    expect(d.getSeconds()).toBeGreaterThanOrEqual(0);
+    expect(d.getSeconds()).toBeLessThanOrEqual(59);
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Date/Date.prototype.getTime.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Date/Date.prototype.getTime.js
@ -0,0 +1,6 @@
+test("basic functionality", () => {
+    var d = new Date();
+    expect(d.getTime()).toBe(d.getTime());
+    expect(d.getTime()).not.toBeNaN();
+    expect(d.getTime()).toBeGreaterThanOrEqual(1580000000000);
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Date/Date.prototype.getUTCDate.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Date/Date.prototype.getUTCDate.js
@ -0,0 +1,7 @@
+test("basic functionality", () => {
+    let d = new Date();
+    expect(d.getUTCDate()).toBe(d.getUTCDate());
+    expect(d.getUTCDate()).not.toBeNaN();
+    expect(d.getUTCDate()).toBeGreaterThanOrEqual(1);
+    expect(d.getUTCDate()).toBeLessThanOrEqual(31);
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Date/Date.prototype.getUTCDay.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Date/Date.prototype.getUTCDay.js
@ -0,0 +1,7 @@
+test("basic functionality", () => {
+    var d = new Date();
+    expect(d.getUTCDay()).toBe(d.getUTCDay());
+    expect(d.getUTCDay()).not.toBeNaN();
+    expect(d.getUTCDay()).toBeGreaterThanOrEqual(0);
+    expect(d.getUTCDay()).toBeLessThanOrEqual(6);
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Date/Date.prototype.getUTCFullYear.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Date/Date.prototype.getUTCFullYear.js
@ -0,0 +1,7 @@
+test("basic functionality", () => {
+    var d = new Date();
+    expect(d.getUTCFullYear()).toBe(d.getUTCFullYear());
+    expect(d.getUTCFullYear()).not.toBeNaN();
+    expect(d.getUTCFullYear()).toBe(d.getUTCFullYear());
+    expect(d.getUTCFullYear()).toBeGreaterThanOrEqual(2020);
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Date/Date.prototype.getUTCHours.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Date/Date.prototype.getUTCHours.js
@ -0,0 +1,7 @@
+test("basic functionality", () => {
+    var d = new Date();
+    expect(d.getUTCHours()).toBe(d.getUTCHours());
+    expect(d.getUTCHours()).not.toBeNaN();
+    expect(d.getUTCHours()).toBeGreaterThanOrEqual(0);
+    expect(d.getUTCHours()).toBeLessThanOrEqual(23);
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Date/Date.prototype.getUTCMilliseconds.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Date/Date.prototype.getUTCMilliseconds.js
@ -0,0 +1,7 @@
+test("basic functionality", () => {
+    var d = new Date();
+    expect(d.getUTCMilliseconds()).toBe(d.getUTCMilliseconds());
+    expect(d.getUTCMilliseconds()).not.toBeNaN();
+    expect(d.getUTCMilliseconds()).toBeGreaterThanOrEqual(0);
+    expect(d.getUTCMilliseconds()).toBeLessThanOrEqual(999);
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Date/Date.prototype.getUTCMinutes.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Date/Date.prototype.getUTCMinutes.js
@ -0,0 +1,7 @@
+test("basic functionality", () => {
+    var d = new Date();
+    expect(d.getUTCMinutes()).toBe(d.getUTCMinutes());
+    expect(d.getUTCMinutes()).not.toBeNaN();
+    expect(d.getUTCMinutes()).toBeGreaterThanOrEqual(0);
+    expect(d.getUTCMinutes()).toBeLessThanOrEqual(59);
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Date/Date.prototype.getUTCMonth.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Date/Date.prototype.getUTCMonth.js
@ -0,0 +1,26 @@
+test("basic functionality", () => {
+    var d = new Date();
+    expect(d.getUTCMonth()).toBe(d.getUTCMonth());
+    expect(d.getUTCMonth()).not.toBeNaN();
+    expect(d.getUTCMonth()).toBeGreaterThanOrEqual(0);
+    expect(d.getUTCMonth()).toBeLessThanOrEqual(11);
+
+    expect(new Date(Date.UTC(2020, 11)).getUTCMonth()).toBe(11);
+});
+
+test("leap years", () => {
+    expect(new Date(Date.UTC(2019, 1, 29)).getUTCDate()).toBe(1);
+    expect(new Date(Date.UTC(2019, 1, 29)).getUTCMonth()).toBe(2);
+    expect(new Date(Date.UTC(2100, 1, 29)).getUTCDate()).toBe(1);
+    expect(new Date(Date.UTC(2100, 1, 29)).getUTCMonth()).toBe(2);
+
+    expect(new Date(Date.UTC(2000, 1, 29)).getUTCDate()).toBe(29);
+    expect(new Date(Date.UTC(2000, 1, 29)).getUTCMonth()).toBe(1);
+    expect(new Date(Date.UTC(2020, 1, 29)).getUTCDate()).toBe(29);
+    expect(new Date(Date.UTC(2020, 1, 29)).getUTCMonth()).toBe(1);
+
+    expect(new Date(Date.UTC(2019, 2, 1)).getUTCDate()).toBe(1);
+    expect(new Date(Date.UTC(2019, 2, 1)).getUTCMonth()).toBe(2);
+    expect(new Date(Date.UTC(2020, 2, 1)).getUTCDate()).toBe(1);
+    expect(new Date(Date.UTC(2020, 2, 1)).getUTCMonth()).toBe(2);
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Date/Date.prototype.getUTCSeconds.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Date/Date.prototype.getUTCSeconds.js
@ -0,0 +1,7 @@
+test("basic functionality", () => {
+    var d = new Date();
+    expect(d.getUTCSeconds()).toBe(d.getUTCSeconds());
+    expect(d.getUTCSeconds()).not.toBeNaN();
+    expect(d.getUTCSeconds()).toBeGreaterThanOrEqual(0);
+    expect(d.getUTCSeconds()).toBeLessThanOrEqual(59);
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Date/Date.prototype.toISOString.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Date/Date.prototype.toISOString.js
@ -0,0 +1,7 @@
+test("basic functionality", () => {
+    expect(new Date(1597955034555).toISOString()).toBe("2020-08-20T20:23:54.555Z");
+    expect(new Date(Date.UTC(22020)).toISOString()).toBe("+022020-01-01T00:00:00.000Z");
+    expect(new Date(Date.UTC(1950)).toISOString()).toBe("1950-01-01T00:00:00.000Z");
+    expect(new Date(Date.UTC(1800)).toISOString()).toBe("1800-01-01T00:00:00.000Z");
+    expect(new Date(Date.UTC(-100)).toISOString()).toBe("-000100-01-01T00:00:00.000Z");
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Error/Error.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Error/Error.js
@ -0,0 +1,18 @@
+test("basic functionality", () => {
+    expect(Error).toHaveLength(1);
+    expect(Error.name).toBe("Error");
+});
+
+test("name", () => {
+    [Error(), Error(undefined), Error("test"), Error(42), Error(null)].forEach(error => {
+        expect(error.name).toBe("Error");
+    });
+});
+
+test("message", () => {
+    expect(Error().message).toBe("");
+    expect(Error(undefined).message).toBe("");
+    expect(Error("test").message).toBe("test");
+    expect(Error(42).message).toBe("42");
+    expect(Error(null).message).toBe("null");
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Error/Error.prototype.name.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Error/Error.prototype.name.js
@ -0,0 +1,10 @@
+test("basic functionality", () => {
+    expect(Error.prototype).not.toHaveProperty("length");
+
+    var changedInstance = new Error("");
+    changedInstance.name = "NewCustomError";
+    expect(changedInstance.name).toBe("NewCustomError");
+
+    var normalInstance = new Error("");
+    expect(normalInstance.name).toBe("Error");
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Error/Error.prototype.toString.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Error/Error.prototype.toString.js
@ -0,0 +1,7 @@
+test("basic functionality", () => {
+    expect(Error().toString()).toBe("Error");
+    expect(Error(undefined).toString()).toBe("Error");
+    expect(Error(null).toString()).toBe("Error: null");
+    expect(Error("test").toString()).toBe("Error: test");
+    expect(Error(42).toString()).toBe("Error: 42");
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Function/Function.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Function/Function.js
@ -0,0 +1,53 @@
+describe("correct behavior", () => {
+    test("constructor properties", () => {
+        expect(Function).toHaveLength(1);
+        expect(Function.name).toBe("Function");
+        expect(Function.prototype).toHaveLength(0);
+        expect(Function.prototype.name).toBe("");
+    });
+
+    test("typeof", () => {
+        expect(typeof Function()).toBe("function");
+        expect(typeof new Function()).toBe("function");
+    });
+
+    test("basic functionality", () => {
+        expect(Function()()).toBeUndefined();
+        expect(new Function()()).toBeUndefined();
+        expect(Function("return 42")()).toBe(42);
+        expect(new Function("return 42")()).toBe(42);
+        expect(new Function("foo", "return foo")(42)).toBe(42);
+        expect(new Function("foo,bar", "return foo + bar")(1, 2)).toBe(3);
+        expect(new Function("foo", "bar", "return foo + bar")(1, 2)).toBe(3);
+        expect(new Function("foo", "bar,baz", "return foo + bar + baz")(1, 2, 3)).toBe(6);
+        expect(new Function("foo", "bar", "baz", "return foo + bar + baz")(1, 2, 3)).toBe(6);
+        expect(new Function("foo", "if (foo) { return 42; } else { return 'bar'; }")(true)).toBe(
+            42
+        );
+        expect(new Function("foo", "if (foo) { return 42; } else { return 'bar'; }")(false)).toBe(
+            "bar"
+        );
+        expect(new Function("return typeof Function()")()).toBe("function");
+        expect(new Function("x", "return function (y) { return x + y };")(1)(2)).toBe(3);
+
+        expect(new Function("-->")()).toBeUndefined();
+
+        expect(new Function().name).toBe("anonymous");
+        expect(new Function().toString()).toBe("function anonymous() {\n  ???\n}");
+    });
+});
+
+describe("errors", () => {
+    test("syntax error", () => {
+        expect(() => {
+            new Function("[");
+        })
+            // This might be confusing at first but keep in mind it's actually parsing
+            // function anonymous() { [ }
+            // This is in line with what other engines are reporting.
+            .toThrowWithMessage(
+                SyntaxError,
+                "Unexpected token CurlyClose. Expected BracketClose (line: 4, column: 1)"
+            );
+    });
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Function/Function.prototype.@@hasInstance.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Function/Function.prototype.@@hasInstance.js
@ -0,0 +1,8 @@
+test("basic functionality", () => {
+    expect(Function.prototype[Symbol.hasInstance]).toHaveLength(1);
+
+    function Foo() {}
+    const foo = new Foo();
+
+    expect(Function.prototype[Symbol.hasInstance].call(Foo, foo)).toBeTrue();
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Function/Function.prototype.apply.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Function/Function.prototype.apply.js
@ -0,0 +1,51 @@
+test("basic functionality", () => {
+    function Foo(arg) {
+        this.foo = arg;
+    }
+    function Bar(arg) {
+        this.bar = arg;
+    }
+    function FooBar(arg) {
+        Foo.apply(this, [arg]);
+        Bar.apply(this, [arg]);
+    }
+    function FooBarBaz(arg) {
+        Foo.apply(this, [arg]);
+        Bar.apply(this, [arg]);
+        this.baz = arg;
+    }
+
+    expect(Function.prototype.apply).toHaveLength(2);
+
+    var foo = new Foo("test");
+    expect(foo.foo).toBe("test");
+    expect(foo.bar).toBeUndefined();
+    expect(foo.baz).toBeUndefined();
+
+    var bar = new Bar("test");
+    expect(bar.foo).toBeUndefined();
+    expect(bar.bar).toBe("test");
+    expect(bar.baz).toBeUndefined();
+
+    var foobar = new FooBar("test");
+    expect(foobar.foo).toBe("test");
+    expect(foobar.bar).toBe("test");
+    expect(foobar.baz).toBeUndefined();
+
+    var foobarbaz = new FooBarBaz("test");
+    expect(foobarbaz.foo).toBe("test");
+    expect(foobarbaz.bar).toBe("test");
+    expect(foobarbaz.baz).toBe("test");
+
+    expect(Math.abs.apply(null, [-1])).toBe(1);
+
+    var add = (x, y) => x + y;
+    expect(add.apply(null, [1, 2])).toBe(3);
+
+    var multiply = function (x, y) {
+        return x * y;
+    };
+    expect(multiply.apply(null, [3, 4])).toBe(12);
+
+    expect((() => this).apply("foo")).toBe(globalThis);
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Function/Function.prototype.bind.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Function/Function.prototype.bind.js
@ -0,0 +1,149 @@
+describe("basic behavior", () => {
+    test("basic binding", () => {
+        expect(Function.prototype.bind).toHaveLength(1);
+
+        var charAt = String.prototype.charAt.bind("bar");
+        expect(charAt(0) + charAt(1) + charAt(2)).toBe("bar");
+
+        function getB() {
+            return this.toUpperCase().charAt(0);
+        }
+        expect(getB.bind("bar")()).toBe("B");
+    });
+
+    test("bound functions work with array functions", () => {
+        var Make3 = Number.bind(null, 3);
+        expect([55].map(Make3)[0]).toBe(3);
+
+        var MakeTrue = Boolean.bind(null, true);
+
+        expect([1, 2, 3].filter(MakeTrue)).toHaveLength(3);
+        expect(
+            [1, 2, 3].reduce(
+                function (acc, x) {
+                    return acc + x;
+                }.bind(null, 4, 5)
+            )
+        ).toBe(9);
+        expect(
+            [1, 2, 3].reduce(
+                function (acc, x) {
+                    return acc + x + this;
+                }.bind(3)
+            )
+        ).toBe(12);
+    });
+});
+
+describe("bound function arguments", () => {
+    function sum(a, b, c) {
+        return a + b + c;
+    }
+    var boundSum = sum.bind(null, 10, 5);
+
+    test("arguments are bound to the function", () => {
+        expect(boundSum()).toBeNaN();
+        expect(boundSum(5)).toBe(20);
+        expect(boundSum(5, 6, 7)).toBe(20);
+    });
+
+    test("arguments are appended to a BoundFunction's bound arguments", () => {
+        expect(boundSum.bind(null, 5)()).toBe(20);
+    });
+
+    test("binding a constructor's arguments", () => {
+        var Make5 = Number.bind(null, 5);
+        expect(Make5()).toBe(5);
+        expect(new Make5().valueOf()).toBe(5);
+    });
+
+    test("length property", () => {
+        expect(sum).toHaveLength(3);
+        expect(boundSum).toHaveLength(1);
+        expect(boundSum.bind(null, 5)).toHaveLength(0);
+        expect(boundSum.bind(null, 5, 6, 7, 8)).toHaveLength(0);
+    });
+});
+
+describe("bound function |this|", () => {
+    function identity() {
+        return this;
+    }
+
+    test("captures global object as |this| if |this| is null or undefined", () => {
+        expect(identity.bind()()).toBe(globalThis);
+        expect(identity.bind(null)()).toBe(globalThis);
+        expect(identity.bind(undefined)()).toBe(globalThis);
+
+        function Foo() {
+            expect(identity.bind()()).toBe(globalThis);
+            expect(identity.bind(this)()).toBe(this);
+        }
+        new Foo();
+    });
+
+    test("does not capture global object as |this| if |this| is null or undefined in strict mode", () => {
+        "use strict";
+
+        function strictIdentity() {
+            return this;
+        }
+
+        expect(strictIdentity.bind()()).toBeUndefined();
+        expect(strictIdentity.bind(null)()).toBeNull();
+        expect(strictIdentity.bind(undefined)()).toBeUndefined();
+    });
+
+    test("primitive |this| values are converted to objects", () => {
+        expect(identity.bind("foo")()).toBeInstanceOf(String);
+        expect(identity.bind(123)()).toBeInstanceOf(Number);
+        expect(identity.bind(true)()).toBeInstanceOf(Boolean);
+    });
+
+    test("bound functions retain |this| values passed to them", () => {
+        var obj = { foo: "bar" };
+        expect(identity.bind(obj)()).toBe(obj);
+    });
+
+    test("bound |this| cannot be changed after being set", () => {
+        expect(identity.bind("foo").bind(123)()).toBeInstanceOf(String);
+    });
+
+    test("arrow functions cannot be bound", () => {
+        expect((() => this).bind("foo")()).toBe(globalThis);
+    });
+});
+
+describe("bound function constructors", () => {
+    function Bar() {
+        this.x = 3;
+        this.y = 4;
+    }
+
+    Bar.prototype.baz = "baz";
+    var BoundBar = Bar.bind({ u: 5, v: 6 });
+    var bar = new BoundBar();
+
+    test("bound |this| value does not affect constructor", () => {
+        expect(bar.x).toBe(3);
+        expect(bar.y).toBe(4);
+        expect(typeof bar.u).toBe("undefined");
+        expect(typeof bar.v).toBe("undefined");
+    });
+
+    test("bound functions retain original prototype", () => {
+        expect(bar.baz).toBe("baz");
+    });
+
+    test("bound functions do not have a prototype property", () => {
+        expect(BoundBar).not.toHaveProperty("prototype");
+    });
+});
+
+describe("errors", () => {
+    test("does not accept non-function values", () => {
+        expect(() => {
+            Function.prototype.bind.call("foo");
+        }).toThrowWithMessage(TypeError, "Not a Function object");
+    });
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Function/Function.prototype.call.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Function/Function.prototype.call.js
@ -0,0 +1,53 @@
+test("length", () => {
+    expect(Function.prototype.call).toHaveLength(1);
+});
+
+test("basic functionality", () => {
+    function Foo(arg) {
+        this.foo = arg;
+    }
+    function Bar(arg) {
+        this.bar = arg;
+    }
+    function FooBar(arg) {
+        Foo.call(this, arg);
+        Bar.call(this, arg);
+    }
+    function FooBarBaz(arg) {
+        Foo.call(this, arg);
+        Bar.call(this, arg);
+        this.baz = arg;
+    }
+
+    var foo = new Foo("test");
+    expect(foo.foo).toBe("test");
+    expect(foo.bar).toBeUndefined();
+    expect(foo.baz).toBeUndefined();
+
+    var bar = new Bar("test");
+    expect(bar.foo).toBeUndefined();
+    expect(bar.bar).toBe("test");
+    expect(bar.baz).toBeUndefined();
+
+    var foobar = new FooBar("test");
+    expect(foobar.foo).toBe("test");
+    expect(foobar.bar).toBe("test");
+    expect(foobar.baz).toBeUndefined();
+
+    var foobarbaz = new FooBarBaz("test");
+    expect(foobarbaz.foo).toBe("test");
+    expect(foobarbaz.bar).toBe("test");
+    expect(foobarbaz.baz).toBe("test");
+
+    expect(Math.abs.call(null, -1)).toBe(1);
+
+    var add = (x, y) => x + y;
+    expect(add.call(null, 1, 2)).toBe(3);
+
+    var multiply = function (x, y) {
+        return x * y;
+    };
+    expect(multiply.call(null, 3, 4)).toBe(12);
+
+    expect((() => this).call("foo")).toBe(globalThis);
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Function/Function.prototype.toString.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Function/Function.prototype.toString.js
@ -0,0 +1,17 @@
+test("basic functionality", () => {
+    expect(function () {}.toString()).toBe("function () {\n  ???\n}");
+    expect(function (foo) {}.toString()).toBe("function (foo) {\n  ???\n}");
+    expect(function (foo, bar, baz) {}.toString()).toBe("function (foo, bar, baz) {\n  ???\n}");
+    expect(
+        function (foo, bar, baz) {
+            if (foo) {
+                return baz;
+            } else if (bar) {
+                return foo;
+            }
+            return bar + 42;
+        }.toString()
+    ).toBe("function (foo, bar, baz) {\n  ???\n}");
+    expect(console.debug.toString()).toBe("function debug() {\n  [NativeFunction]\n}");
+    expect(Function.toString()).toBe("function Function() {\n  [FunctionConstructor]\n}");
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Infinity/Infinity.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Infinity/Infinity.js
@ -0,0 +1,22 @@
+test("basic functionality", () => {
+    expect(Infinity + "").toBe("Infinity");
+    expect(-Infinity + "").toBe("-Infinity");
+    expect(Infinity).toBe(Infinity);
+    expect(Infinity - 1).toBe(Infinity);
+    expect(Infinity + 1).toBe(Infinity);
+    expect(-Infinity).toBe(-Infinity);
+    expect(-Infinity - 1).toBe(-Infinity);
+    expect(-Infinity + 1).toBe(-Infinity);
+    expect(1 / Infinity).toBe(0);
+    expect(1 / -Infinity).toBe(-0);
+    expect(1 / 0).toBe(Infinity);
+    expect(-1 / 0).toBe(-Infinity);
+    expect(-100).toBeLessThan(Infinity);
+    expect(0).toBeLessThan(Infinity);
+    expect(100).toBeLessThan(Infinity);
+    expect(-Infinity).toBeLessThan(Infinity);
+    expect(-100).toBeGreaterThan(-Infinity);
+    expect(0).toBeGreaterThan(-Infinity);
+    expect(100).toBeGreaterThan(-Infinity);
+    expect(Infinity).toBeGreaterThan(-Infinity);
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/JSON/JSON.@@toStringTag.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/JSON/JSON.@@toStringTag.js
@ -0,0 +1,4 @@
+test("basic functionality", () => {
+    expect(JSON[Symbol.toStringTag]).toBe("JSON");
+    expect(JSON.toString()).toBe("[object JSON]");
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/JSON/JSON.parse-reviver.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/JSON/JSON.parse-reviver.js
@ -0,0 +1,11 @@
+test("basic functionality", () => {
+    let string = `{"var1":10,"var2":"hello","var3":{"nested":5}}`;
+
+    let object = JSON.parse(string, (key, value) =>
+        typeof value === "number" ? value * 2 : value
+    );
+    expect(object).toEqual({ var1: 20, var2: "hello", var3: { nested: 10 } });
+
+    object = JSON.parse(string, (key, value) => (typeof value === "number" ? undefined : value));
+    expect(object).toEqual({ var2: "hello", var3: {} });
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/JSON/JSON.parse.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/JSON/JSON.parse.js
@ -0,0 +1,37 @@
+test("basic functionality", () => {
+    expect(JSON.parse).toHaveLength(2);
+
+    const properties = [
+        ["5", 5],
+        ["null", null],
+        ["true", true],
+        ["false", false],
+        ['"test"', "test"],
+        ['[1,2,"foo"]', [1, 2, "foo"]],
+        ['{"foo":1,"bar":"baz"}', { foo: 1, bar: "baz" }],
+    ];
+
+    properties.forEach(testCase => {
+        expect(JSON.parse(testCase[0])).toEqual(testCase[1]);
+    });
+});
+
+test("syntax errors", () => {
+    [
+        undefined,
+        NaN,
+        -NaN,
+        Infinity,
+        -Infinity,
+        '{ "foo" }',
+        '{ foo: "bar" }',
+        "[1,2,3,]",
+        "[1,2,3, ]",
+        '{ "foo": "bar",}',
+        '{ "foo": "bar", }',
+    ].forEach(test => {
+        expect(() => {
+            JSON.parse(test);
+        }).toThrow(SyntaxError);
+    });
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/JSON/JSON.stringify-exception-in-property-getter.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/JSON/JSON.stringify-exception-in-property-getter.js
@ -0,0 +1,10 @@
+test("Issue #3548, exception in property getter with replacer function", () => {
+    const o = {
+        get foo() {
+            throw Error();
+        },
+    };
+    expect(() => {
+        JSON.stringify(o, (_, value) => value);
+    }).toThrow(Error);
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/JSON/JSON.stringify-order.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/JSON/JSON.stringify-order.js
@ -0,0 +1,30 @@
+test("basic functionality", () => {
+    let o = {
+        key1: "key1",
+        key2: "key2",
+        key3: "key3",
+    };
+
+    Object.defineProperty(o, "defined", {
+        enumerable: true,
+        get() {
+            o.prop = "prop";
+            return "defined";
+        },
+    });
+
+    o.key4 = "key4";
+
+    o[2] = 2;
+    o[0] = 0;
+    o[1] = 1;
+
+    delete o.key1;
+    delete o.key3;
+
+    o.key1 = "key1";
+
+    expect(JSON.stringify(o)).toBe(
+        '{"0":0,"1":1,"2":2,"key2":"key2","defined":"defined","key4":"key4","key1":"key1"}'
+    );
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/JSON/JSON.stringify-proxy.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/JSON/JSON.stringify-proxy.js
@ -0,0 +1,11 @@
+test("basic functionality", () => {
+    let p = new Proxy([], {
+        get(_, key) {
+            if (key === "length") return 3;
+            return Number(key);
+        },
+    });
+
+    expect(JSON.stringify(p)).toBe("[0,1,2]");
+    expect(JSON.stringify([[new Proxy(p, {})]])).toBe("[[[0,1,2]]]");
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/JSON/JSON.stringify-replacer.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/JSON/JSON.stringify-replacer.js
@ -0,0 +1,38 @@
+test("basic functionality", () => {
+    let o = {
+        var1: "foo",
+        var2: 42,
+        arr: [
+            1,
+            2,
+            {
+                nested: {
+                    hello: "world",
+                },
+                get x() {
+                    return 10;
+                },
+            },
+        ],
+        obj: {
+            subarr: [3],
+        },
+    };
+
+    let string = JSON.stringify(o, (key, value) => {
+        if (key === "hello") return undefined;
+        if (value === 10) return 20;
+        if (key === "subarr") return [3, 4, 5];
+        return value;
+    });
+
+    expect(string).toBe(
+        '{"var1":"foo","var2":42,"arr":[1,2,{"nested":{},"x":20}],"obj":{"subarr":[3,4,5]}}'
+    );
+
+    string = JSON.stringify(o, ["var1", "var1", "var2", "obj"]);
+    expect(string).toBe('{"var1":"foo","var2":42,"obj":{}}');
+
+    string = JSON.stringify(o, ["var1", "var1", "var2", "obj", "subarr"]);
+    expect(string).toBe('{"var1":"foo","var2":42,"obj":{"subarr":[3]}}');
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/JSON/JSON.stringify-space.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/JSON/JSON.stringify-space.js
@ -0,0 +1,47 @@
+test("basic functionality", () => {
+    let o = {
+        foo: 1,
+        bar: "baz",
+        qux: {
+            get x() {
+                return 10;
+            },
+            y() {
+                return 20;
+            },
+            arr: [1, 2, 3],
+        },
+    };
+
+    let string = JSON.stringify(o, null, 4);
+    let expected = `{
+    "foo": 1,
+    "bar": "baz",
+    "qux": {
+        "x": 10,
+        "arr": [
+            1,
+            2,
+            3
+        ]
+    }
+}`;
+
+    expect(string).toBe(expected);
+
+    string = JSON.stringify(o, null, "abcd");
+    expected = `{
+abcd"foo": 1,
+abcd"bar": "baz",
+abcd"qux": {
+abcdabcd"x": 10,
+abcdabcd"arr": [
+abcdabcdabcd1,
+abcdabcdabcd2,
+abcdabcdabcd3
+abcdabcd]
+abcd}
+}`;
+
+    expect(string).toBe(expected);
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/JSON/JSON.stringify.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/JSON/JSON.stringify.js
@ -0,0 +1,71 @@
+describe("correct behavior", () => {
+    test("length", () => {
+        expect(JSON.stringify).toHaveLength(3);
+    });
+
+    test("basic functionality", () => {
+        [
+            [5, "5"],
+            [undefined, undefined],
+            [null, "null"],
+            [NaN, "null"],
+            [-NaN, "null"],
+            [Infinity, "null"],
+            [-Infinity, "null"],
+            [true, "true"],
+            [false, "false"],
+            ["test", '"test"'],
+            [new Number(5), "5"],
+            [new Boolean(false), "false"],
+            [new String("test"), '"test"'],
+            [() => {}, undefined],
+            [[1, 2, "foo"], '[1,2,"foo"]'],
+            [{ foo: 1, bar: "baz", qux() {} }, '{"foo":1,"bar":"baz"}'],
+            [
+                {
+                    var1: 1,
+                    var2: 2,
+                    toJSON(key) {
+                        let o = this;
+                        o.var2 = 10;
+                        return o;
+                    },
+                },
+                '{"var1":1,"var2":10}',
+            ],
+        ].forEach(testCase => {
+            expect(JSON.stringify(testCase[0])).toEqual(testCase[1]);
+        });
+    });
+
+    test("ignores non-enumerable properties", () => {
+        let o = { foo: "bar" };
+        Object.defineProperty(o, "baz", { value: "qux", enumerable: false });
+        expect(JSON.stringify(o)).toBe('{"foo":"bar"}');
+    });
+});
+
+describe("errors", () => {
+    test("cannot serialize BigInt", () => {
+        expect(() => {
+            JSON.stringify(5n);
+        }).toThrow(TypeError, "Cannot serialize BigInt value to JSON");
+    });
+
+    test("cannot serialize circular structures", () => {
+        let bad1 = {};
+        bad1.foo = bad1;
+        let bad2 = [];
+        bad2[5] = [[[bad2]]];
+
+        let bad3a = { foo: "bar" };
+        let bad3b = [1, 2, bad3a];
+        bad3a.bad = bad3b;
+
+        [bad1, bad2, bad3a].forEach(bad => {
+            expect(() => {
+                JSON.stringify(bad);
+            }).toThrow(TypeError, "Cannot stringify circular object");
+        });
+    });
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Math/Math-constants.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Math/Math-constants.js
@ -0,0 +1,10 @@
+test("basic functionality", () => {
+    expect(Math.E).toBeCloseTo(2.718281);
+    expect(Math.LN2).toBeCloseTo(0.693147);
+    expect(Math.LN10).toBeCloseTo(2.302585);
+    expect(Math.LOG2E).toBeCloseTo(1.442695);
+    expect(Math.LOG10E).toBeCloseTo(0.434294);
+    expect(Math.PI).toBeCloseTo(3.1415926);
+    expect(Math.SQRT1_2).toBeCloseTo(0.707106);
+    expect(Math.SQRT2).toBeCloseTo(1.414213);
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Math/Math.@@toStringTag.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Math/Math.@@toStringTag.js
@ -0,0 +1,4 @@
+test("basic functionality", () => {
+    expect(Math[Symbol.toStringTag]).toBe("Math");
+    expect(Math.toString()).toBe("[object Math]");
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Math/Math.abs.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Math/Math.abs.js
@ -0,0 +1,14 @@
+test("basic functionality", () => {
+    expect(Math.abs).toHaveLength(1);
+
+    expect(Math.abs("-1")).toBe(1);
+    expect(Math.abs(-2)).toBe(2);
+    expect(Math.abs(null)).toBe(0);
+    expect(Math.abs("")).toBe(0);
+    expect(Math.abs([])).toBe(0);
+    expect(Math.abs([2])).toBe(2);
+    expect(Math.abs([1, 2])).toBeNaN();
+    expect(Math.abs({})).toBeNaN();
+    expect(Math.abs("string")).toBeNaN();
+    expect(Math.abs()).toBeNaN();
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Math/Math.acosh.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Math/Math.acosh.js
@ -0,0 +1,9 @@
+test("basic functionality", () => {
+    expect(Math.acosh).toHaveLength(1);
+
+    expect(Math.acosh(-1)).toBeNaN();
+    expect(Math.acosh(0)).toBeNaN();
+    expect(Math.acosh(0.5)).toBeNaN();
+    expect(Math.acosh(1)).toBeCloseTo(0);
+    expect(Math.acosh(2)).toBeCloseTo(1.316957);
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Math/Math.asin.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Math/Math.asin.js
@ -0,0 +1,15 @@
+test("basic functionality", () => {
+    expect(Math.asin).toHaveLength(1);
+
+    expect(Math.asin(0)).toBe(0);
+    expect(Math.asin(null)).toBe(0);
+    expect(Math.asin("")).toBe(0);
+    expect(Math.asin([])).toBe(0);
+    // FIXME(LibM): expect(Math.asin(1)).toBeCloseTo(1.5707963267948966);
+    // FIXME(LibM): expect(Math.asin(-1)).toBeCloseTo(-1.5707963267948966);
+    expect(Math.asin()).toBeNaN();
+    expect(Math.asin(undefined)).toBeNaN();
+    expect(Math.asin([1, 2, 3])).toBeNaN();
+    expect(Math.asin({})).toBeNaN();
+    expect(Math.asin("foo")).toBeNaN();
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Math/Math.asinh.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Math/Math.asinh.js
@ -0,0 +1,6 @@
+test("basic functionality", () => {
+    expect(Math.asinh).toHaveLength(1);
+
+    expect(Math.asinh(0)).toBeCloseTo(0);
+    expect(Math.asinh(1)).toBeCloseTo(0.881373);
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Math/Math.atan.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Math/Math.atan.js
@ -0,0 +1,12 @@
+test("basic functionality", () => {
+    expect(Math.atan).toHaveLength(1);
+
+    expect(Math.atan(0)).toBe(0);
+    expect(Math.atan(-0)).toBe(-0);
+    expect(Math.atan(NaN)).toBeNaN();
+    expect(Math.atan(-2)).toBeCloseTo(-1.1071487177940904);
+    expect(Math.atan(2)).toBeCloseTo(1.1071487177940904);
+    expect(Math.atan(Infinity)).toBeCloseTo(Math.PI / 2);
+    expect(Math.atan(-Infinity)).toBeCloseTo(-Math.PI / 2);
+    expect(Math.atan(0.5)).toBeCloseTo(0.4636476090008061);
+});
--- a/Userland/Libraries/LibJS/Tests/builtins/Math/Math.atan2.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Math/Math.atan2.js
@ -0,0 +1,28 @@
+test("basic functionality", () => {
+    expect(Math.atan2).toHaveLength(2);
+
+    expect(Math.atan2(90, 15)).toBeCloseTo(1.4056476493802699);
+    expect(Math.atan2(15, 90)).toBeCloseTo(0.16514867741462683);
+    expect(Math.atan2(+0, -0)).toBeCloseTo(Math.PI);
+    expect(Math.atan2(-0, -0)).toBeCloseTo(-Math.PI);
+    expect(Math.atan2(+0, +0)).toBe(0);
+    expect(Math.atan2(-0, +0)).toBe(-0);
+    expect(Math.atan2(+0, -1)).toBeCloseTo(Math.PI);
+    expect(Math.atan2(-0, -1)).toBeCloseTo(-Math.PI);
+    expect(Math.atan2(+0, 1)).toBe(0);
+    expect(Math.atan2(-0, 1)).toBe(-0);
+    expect(Math.atan2(-1, +0)).toBeCloseTo(-Math.PI / 2);
+    expect(Math.atan2(-1, -0)).toBeCloseTo(-Math.PI / 2);
+    expect(Math.atan2(1, +0)).toBeCloseTo(Math.PI / 2);
+    expect(Math.atan2(1, -0)).toBeCloseTo(Math.PI / 2);
+    expect(Math.atan2(1, -Infinity)).toBeCloseTo(Math.PI);
+    expect(Math.atan2(-1, -Infinity)).toBeCloseTo(-Math.PI);
+    expect(Math.atan2(1, +Infinity)).toBe(0);
+    expect(Math.atan2(-1, +Infinity)).toBe(-0);
+    expect(Math.atan2(+Infinity, 1)).toBeCloseTo(Math.PI / 2);
+    expect(Math.atan2(-Infinity, 1)).toBeCloseTo(-Math.PI / 2);
+    expect(Math.atan2(+Infinity, -Infinity)).toBeCloseTo((3 * Math.PI) / 4);
+    expect(Math.atan2(-Infinity, -Infinity)).toBeCloseTo((-3 * Math.PI) / 4);
+    expect(Math.atan2(+Infinity, +Infinity)).toBeCloseTo(Math.PI / 4);
+    expect(Math.atan2(-Infinity, +Infinity)).toBeCloseTo(-Math.PI / 4);
+});
--- a/Show more
+++ b/Show more