LibJS: Add String Iterator tests

Author: https://github.com/mattco98
Commit: https://github.com/SerenityOS/serenity/commit/4c3a415dc32
Pull-request: https://github.com/SerenityOS/serenity/pull/2780

Libraries/LibJS/Tests/iterators/array-iterator.js

@@ -1,5 +1,5 @@
 test("length", () => {
-    expect(Array.prototype[Symbol.iterator].length).toBe(0);
+    expect(Array.prototype[Symbol.iterator]).toHaveLength(0);
 });
 
 test("@@toStringTag", () => {

Libraries/LibJS/Tests/iterators/string-iterator.js

@@ -0,0 +1,48 @@
+test("length", () => {
+    expect(String.prototype[Symbol.iterator]).toHaveLength(0);
+});
+
+test("basic functionality", () => {
+    const s = "abcd";
+    const it = s[Symbol.iterator]();
+    expect(it.next()).toEqual({ value: "a", done: false });
+    expect(it.next()).toEqual({ value: "b", done: false });
+    expect(it.next()).toEqual({ value: "c", done: false });
+    expect(it.next()).toEqual({ value: "d", 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("casts |this| to string", () => {
+    const it = String.prototype[Symbol.iterator].call(45);
+    expect(it.next()).toEqual({ value: "4", done: false });
+    expect(it.next()).toEqual({ value: "5", done: false });
+    expect(it.next()).toEqual({ value: undefined, done: true });
+
+    const it = String.prototype[Symbol.iterator].call(false);
+    expect(it.next()).toEqual({ value: "f", done: false });
+    expect(it.next()).toEqual({ value: "a", done: false });
+    expect(it.next()).toEqual({ value: "l", done: false });
+    expect(it.next()).toEqual({ value: "s", done: false });
+    expect(it.next()).toEqual({ value: "e", done: false });
+    expect(it.next()).toEqual({ value: undefined, done: true });
+
+    expect(() => {
+        String.prototype[Symbol.iterator].call(null);
+    }).toThrowWithMessage(TypeError, "ToObject on null or undefined");
+    expect(() => {
+        String.prototype[Symbol.iterator].call(undefined);
+    }).toThrowWithMessage(TypeError, "ToObject on null or undefined");
+});
+
+test("utf8 compatible", () => {
+    const it = "ab\u{1f41e}cde"[Symbol.iterator]();
+    expect(it.next()).toEqual({ value: "a", done: false });
+    expect(it.next()).toEqual({ value: "b", done: false });
+    expect(it.next()).toEqual({ value: "🐞", done: false });
+    expect(it.next()).toEqual({ value: "c", done: false });
+    expect(it.next()).toEqual({ value: "d", done: false });
+    expect(it.next()).toEqual({ value: "e", done: false });
+    expect(it.next()).toEqual({ value: undefined, done: true });
+});