LibJS: Implement Array.prototype.groupBy

Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype-generic-functions.js

@@ -304,4 +304,17 @@ describe("ability to work with generic non-array objects", () => {
             2,
         ]);
     });
+
+    test("groupBy", () => {
+        const visited = [];
+        const o = { length: 5, 0: "foo", 1: "bar", 3: "baz" };
+        const result = Array.prototype.groupBy.call(o, (value, _, object) => {
+            expect(object).toBe(o);
+            visited.push(value);
+            return value !== undefined ? value.startsWith("b") : false;
+        });
+        expect(visited).toEqual(["foo", "bar", undefined, "baz", undefined]);
+        expect(result.false).toEqual(["foo", undefined, undefined]);
+        expect(result.true).toEqual(["bar", "baz"]);
+    });
 });

Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.groupBy.js

@@ -0,0 +1,101 @@
+test("length is 1", () => {
+    expect(Array.prototype.groupBy).toHaveLength(1);
+});
+
+describe("errors", () => {
+    test("callback must be a function", () => {
+        expect(() => {
+            [].groupBy(undefined);
+        }).toThrowWithMessage(TypeError, "undefined is not a function");
+    });
+
+    test("null or undefined this value", () => {
+        expect(() => {
+            Array.prototype.groupBy.call();
+        }).toThrowWithMessage(TypeError, "ToObject on null or undefined");
+
+        expect(() => {
+            Array.prototype.groupBy.call(undefined);
+        }).toThrowWithMessage(TypeError, "ToObject on null or undefined");
+
+        expect(() => {
+            Array.prototype.groupBy.call(null);
+        }).toThrowWithMessage(TypeError, "ToObject on null or undefined");
+    });
+});
+
+describe("normal behavior", () => {
+    test("basic functionality", () => {
+        const array = [1, 2, 3, 4, 5, 6];
+        const visited = [];
+
+        const firstResult = array.groupBy(value => {
+            visited.push(value);
+            return value % 2 === 0;
+        });
+
+        expect(visited).toEqual([1, 2, 3, 4, 5, 6]);
+        expect(firstResult.true).toEqual([2, 4, 6]);
+        expect(firstResult.false).toEqual([1, 3, 5]);
+
+        const firstKeys = Object.keys(firstResult);
+        expect(firstKeys).toHaveLength(2);
+        expect(firstKeys[0]).toBe("false");
+        expect(firstKeys[1]).toBe("true");
+
+        const secondResult = array.groupBy((_, index) => {
+            return index < array.length / 2;
+        });
+
+        expect(secondResult.true).toEqual([1, 2, 3]);
+        expect(secondResult.false).toEqual([4, 5, 6]);
+
+        const secondKeys = Object.keys(secondResult);
+        expect(secondKeys).toHaveLength(2);
+        expect(secondKeys[0]).toBe("true");
+        expect(secondKeys[1]).toBe("false");
+
+        const thisArg = [7, 8, 9, 10, 11, 12];
+        const thirdResult = array.groupBy(function (_, __, arrayVisited) {
+            expect(arrayVisited).toBe(array);
+            expect(this).toBe(thisArg);
+        }, thisArg);
+
+        expect(thirdResult.undefined).not.toBe(array);
+        expect(thirdResult.undefined).not.toBe(thisArg);
+        expect(thirdResult.undefined).toEqual(array);
+
+        const thirdKeys = Object.keys(thirdResult);
+        expect(thirdKeys).toHaveLength(1);
+        expect(thirdKeys[0]).toBe("undefined");
+    });
+
+    test("is unscopable", () => {
+        expect(Array.prototype[Symbol.unscopables].groupBy).toBeTrue();
+        const array = [];
+        with (array) {
+            expect(() => {
+                groupBy;
+            }).toThrowWithMessage(ReferenceError, "'groupBy' is not defined");
+        }
+    });
+
+    test("never calls callback with empty array", () => {
+        var callbackCalled = 0;
+        expect(
+            [].groupBy(() => {
+                callbackCalled++;
+            })
+        ).toEqual({});
+        expect(callbackCalled).toBe(0);
+    });
+
+    test("calls callback once for every item", () => {
+        var callbackCalled = 0;
+        const result = [1, 2, 3].groupBy(() => {
+            callbackCalled++;
+        });
+        expect(result.undefined).toEqual([1, 2, 3]);
+        expect(callbackCalled).toBe(3);
+    });
+});