LibJS: Implement String.prototype.isWellFormed

Userland/Libraries/LibJS/Tests/builtins/String/String.prototype.isWellFormed.js

@@ -0,0 +1,47 @@
+describe("errors", () => {
+    test("called with value that cannot be converted to a string", () => {
+        expect(() => {
+            String.prototype.isWellFormed.call(Symbol.hasInstance);
+        }).toThrowWithMessage(TypeError, "Cannot convert symbol to string");
+    });
+});
+
+describe("basic functionality", () => {
+    test("ascii strings", () => {
+        expect("".isWellFormed()).toBeTrue();
+        expect("foo".isWellFormed()).toBeTrue();
+        expect("abcdefghi".isWellFormed()).toBeTrue();
+    });
+
+    test("valid UTF-16 strings", () => {
+        expect("😀".isWellFormed()).toBeTrue();
+        expect("\ud83d\ude00".isWellFormed()).toBeTrue();
+    });
+
+    test("invalid UTF-16 strings", () => {
+        expect("😀".slice(0, 1).isWellFormed()).toBeFalse();
+        expect("😀".slice(1, 2).isWellFormed()).toBeFalse();
+        expect("\ud83d".isWellFormed()).toBeFalse();
+        expect("\ude00".isWellFormed()).toBeFalse();
+        expect("a\ud83d".isWellFormed()).toBeFalse();
+        expect("a\ude00".isWellFormed()).toBeFalse();
+        expect("\ud83da".isWellFormed()).toBeFalse();
+        expect("\ude00a".isWellFormed()).toBeFalse();
+        expect("a\ud83da".isWellFormed()).toBeFalse();
+        expect("a\ude00a".isWellFormed()).toBeFalse();
+    });
+
+    test("object converted to a string", () => {
+        let toStringCalled = false;
+
+        const obj = {
+            toString: function () {
+                toStringCalled = true;
+                return "toString";
+            },
+        };
+
+        expect(String.prototype.isWellFormed.call(obj)).toBeTrue();
+        expect(toStringCalled).toBeTrue();
+    });
+});