LibJS: Implement Array.prototype.copyWithin generically

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

@@ -130,6 +130,35 @@ describe("ability to work with generic non-array objects", () => {
         }
     });
 
+    test("copyWithin", () => {
+        const initial_o = { length: 5, 0: "foo", 1: "bar", 3: "baz" };
+        {
+            const o = { length: 5, 0: "foo", 1: "bar", 3: "baz" };
+            // returns value and modifies
+            expect(Array.prototype.copyWithin.call(o, 0, 0)).toEqual(o);
+            expect(o).toEqual(initial_o);
+        }
+
+        {
+            const o = {};
+            expect(Array.prototype.copyWithin.call(o, 1, 16, 32)).toEqual(o);
+            expect(o).toEqual({});
+        }
+
+        {
+            const o = { length: 100 };
+            expect(Array.prototype.copyWithin.call(o, 1, 16, 32)).toEqual(o);
+            expect(o).toEqual({ length: 100 });
+        }
+
+        {
+            const o = { length: 5, 0: "foo", 1: "bar", 3: "baz" };
+            // returns value and modifies
+            expect(Array.prototype.copyWithin.call(o, 2, 0)).toEqual(o);
+            expect(o).toEqual({ length: 5, 0: "foo", 1: "bar", 2: "foo", 3: "bar" });
+        }
+    });
+
     const o = { length: 5, 0: "foo", 1: "bar", 3: "baz" };
 
     test("every", () => {

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

@@ -0,0 +1,45 @@
+test("length is 2", () => {
+    expect(Array.prototype.copyWithin).toHaveLength(2);
+});
+
+describe("normal behavior", () => {
+    test("Noop", () => {
+        var array = [1, 2];
+        array.copyWithin(0, 0);
+        expect(array).toEqual([1, 2]);
+    });
+
+    test("basic behavior", () => {
+        var array = [1, 2, 3];
+
+        var b = array.copyWithin(1, 2);
+        expect(b).toEqual(array);
+        expect(array).toEqual([1, 3, 3]);
+
+        b = array.copyWithin(2, 0);
+        expect(b).toEqual(array);
+        expect(array).toEqual([1, 3, 1]);
+    });
+
+    test("start > target", () => {
+        var array = [1, 2, 3];
+        var b = array.copyWithin(0, 1);
+        expect(b).toEqual(array);
+        expect(array).toEqual([2, 3, 3]);
+    });
+
+    test("overwriting behavior", () => {
+        var array = [1, 2, 3];
+        var b = array.copyWithin(1, 0);
+        expect(b).toEqual(array);
+        expect(array).toEqual([1, 1, 2]);
+    });
+
+    test("specify end", () => {
+        var array = [1, 2, 3];
+
+        b = array.copyWithin(2, 0, 1);
+        expect(b).toEqual(array);
+        expect(array).toEqual([1, 2, 1]);
+    });
+});