src/platform/polyfills.ts

@@ -15,3 +15,24 @@ if (typeof window.crypto.randomUUID === "undefined") {
       ) as `${string}-${string}-${string}-${string}-${string}`;
     };
 }
+
+
+if (typeof Promise.withResolvers === "undefined") {
+  // Chrome/Edge 119, Firefox 121, Safari/iOS 17.4
+
+  // Promise.withResolvers is generic and works with subclasses - the typescript built-in decl
+  // could not handle the subclassing case.
+  Promise.withResolvers = function <T>(this: AnyPromiseConstructor<T>) {
+    let resolve!: PromiseWithResolvers<T>["resolve"], reject!: PromiseWithResolvers<T>["reject"];
+    // These variables are expected to be set after `new this()`
+
+    const promise = new this((resolve0, reject0) => {
+      resolve = resolve0;
+      reject = reject0;
+    })
+
+    return {
+      promise, resolve, reject
+    }
+  }
+}

types/lib.esnext.promise.d.ts

@@ -0,0 +1,26 @@
+interface AnyPromiseWithResolvers<T, Instance> {
+  promise: Instance;
+  resolve: (value: T | PromiseLike<T>) => void;
+  reject: (reason?: any) => void;
+}
+
+type AnyPromiseConstructor<T> = new (
+  executor: (
+    resolve: PromiseWithResolvers<T>["resolve"],
+    reject: PromiseWithResolvers<T>["reject"],
+  ) => void,
+) => Promise<T>;
+
+interface PromiseConstructor {
+  /**
+   * Creates a new Promise and returns it in an object, along with its resolve and reject functions.
+   * @returns An object with the properties `promise`, `resolve`, and `reject`.
+   *
+   * ```ts
+   * const { promise, resolve, reject } = Promise.withResolvers<T>();
+   * ```
+   */
+  withResolvers<T, K extends AnyPromiseConstructor<T>>(
+    this: K,
+  ): AnyPromiseWithResolvers<T, InstanceType<K>>;
+}