LibJS: Add Array.prototype.keys()

Author: https://github.com/davidot
Commit: e044a3e428
Pull-request: https://github.com/SerenityOS/serenity/pull/8020
Reviewed-by: https://github.com/linusg

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

@@ -0,0 +1,44 @@
+test("length", () => {
+    expect(Array.prototype.keys.length).toBe(0);
+});
+
+test("basic functionality", () => {
+    const a = ["a", "b", "c"];
+    const it = a.keys();
+    expect(it.next()).toEqual({ value: 0, done: false });
+    expect(it.next()).toEqual({ value: 1, done: false });
+    expect(it.next()).toEqual({ value: 2, 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 = [].keys.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 = ["a", "b"];
+    const it = a.keys();
+    expect(it.next()).toEqual({ value: 0, done: false });
+    expect(it.next()).toEqual({ value: 1, done: false });
+    a.push("c");
+    expect(it.next()).toEqual({ value: 2, 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 = ["a", "b"];
+    const it = a.keys();
+    expect(it.next()).toEqual({ value: 0, done: false });
+    expect(it.next()).toEqual({ value: 1, done: false });
+    expect(it.next()).toEqual({ value: undefined, done: true });
+    a.push("c");
+    expect(it.next()).toEqual({ value: undefined, done: true });
+});