LibJS: Implement the Promise.withResolvers proposal

https://github.com/tc39/proposal-promise-with-resolvers

Userland/Libraries/LibJS/Tests/builtins/Promise/Promise.withResolvers.js

@@ -0,0 +1,56 @@
+describe("errors", () => {
+    test("this value must be a constructor", () => {
+        expect(() => {
+            Promise.withResolvers.call(Symbol.hasInstance);
+        }).toThrowWithMessage(TypeError, "Symbol(Symbol.hasInstance) is not a constructor");
+    });
+});
+
+describe("normal behavior", () => {
+    test("length is 0", () => {
+        expect(Promise.withResolvers).toHaveLength(0);
+    });
+
+    test("returned promise is a Promise", () => {
+        const { promise, resolve, reject } = Promise.withResolvers();
+        expect(promise).toBeInstanceOf(Promise);
+    });
+
+    test("returned resolve/reject are unary functions", () => {
+        const { promise, resolve, reject } = Promise.withResolvers();
+
+        expect(resolve).toBeInstanceOf(Function);
+        expect(resolve).toHaveLength(1);
+
+        expect(reject).toBeInstanceOf(Function);
+        expect(reject).toHaveLength(1);
+    });
+
+    test("returned promise can be resolved", () => {
+        const { promise, resolve, reject } = Promise.withResolvers();
+
+        let fulfillmentValue = null;
+        promise.then(value => {
+            fulfillmentValue = value;
+        });
+
+        resolve("Some value");
+        runQueuedPromiseJobs();
+
+        expect(fulfillmentValue).toBe("Some value");
+    });
+
+    test("returned promise can be rejected", () => {
+        const { promise, resolve, reject } = Promise.withResolvers();
+
+        let rejectionReason = null;
+        promise.catch(value => {
+            rejectionReason = value;
+        });
+
+        reject("Some value");
+        runQueuedPromiseJobs();
+
+        expect(rejectionReason).toBe("Some value");
+    });
+});