LibJS: Re-implement the Array Grouping proposal as static methods

Closes #19495.

Userland/Libraries/LibJS/Tests/builtins/Object/Object.groupBy.js

@@ -0,0 +1,77 @@
+test("length is 2", () => {
+    expect(Object.groupBy).toHaveLength(2);
+});
+
+describe("errors", () => {
+    test("callback must be a function", () => {
+        expect(() => {
+            Object.groupBy([], undefined);
+        }).toThrowWithMessage(TypeError, "undefined is not a function");
+    });
+
+    test("null or undefined items value", () => {
+        expect(() => {
+            Object.groupBy();
+        }).toThrowWithMessage(TypeError, "undefined cannot be converted to an object");
+
+        expect(() => {
+            Object.groupBy(undefined);
+        }).toThrowWithMessage(TypeError, "undefined cannot be converted to an object");
+
+        expect(() => {
+            Object.groupBy(null);
+        }).toThrowWithMessage(TypeError, "null cannot be converted to an object");
+    });
+});
+
+describe("normal behavior", () => {
+    test("basic functionality", () => {
+        const array = [1, 2, 3, 4, 5, 6];
+        const visited = [];
+
+        const firstResult = Object.groupBy(array, 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 = Object.groupBy(array, (_, 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");
+    });
+
+    test("never calls callback with empty array", () => {
+        var callbackCalled = 0;
+        expect(
+            Object.groupBy([], () => {
+                callbackCalled++;
+            })
+        ).toEqual({});
+        expect(callbackCalled).toBe(0);
+    });
+
+    test("calls callback once for every item", () => {
+        var callbackCalled = 0;
+        const result = Object.groupBy([1, 2, 3], () => {
+            callbackCalled++;
+        });
+        expect(result.undefined).toEqual([1, 2, 3]);
+        expect(callbackCalled).toBe(3);
+    });
+});