LibJS: Implement parsing and execution of optional chains

Userland/Libraries/LibJS/Tests/syntax/optional-chaining.js

@@ -0,0 +1,40 @@
+test("parse optional-chaining", () => {
+    expect(`a?.b`).toEval();
+    expect(`a?.4:.5`).toEval();
+    expect(`a?.[b]`).toEval();
+    expect(`a?.b[b]`).toEval();
+    expect(`a?.b(c)`).toEval();
+    expect(`a?.b?.(c, d)`).toEval();
+    expect(`a?.b?.()`).toEval();
+    expect("a?.b``").not.toEval();
+    expect("a?.b?.``").not.toEval();
+    expect("new Foo?.bar").not.toEval();
+    expect("new (Foo?.bar)").toEval();
+    // FIXME: This should pass.
+    // expect("(new Foo)?.bar").toEval();
+});
+
+test("evaluate optional-chaining", () => {
+    for (let nullishObject of [null, undefined]) {
+        expect((() => nullishObject?.b)()).toBeUndefined();
+    }
+
+    expect(
+        (() => {
+            let a = {};
+            return a?.foo?.bar?.baz;
+        })()
+    ).toBeUndefined();
+
+    expect(
+        (() => {
+            let a = { foo: { bar: () => 42 } };
+            return `${a?.foo?.bar?.()}-${a?.foo?.baz?.()}`;
+        })()
+    ).toBe("42-undefined");
+
+    expect(() => {
+        let a = { foo: { bar: () => 42 } };
+        return a.foo?.baz.nonExistentProperty;
+    }).toThrow();
+});