PendingRequest: a way to implement Promise-based APIs across process boundaries like iframes or ServiceWorkers.

PendingRequest.ts

import {Deferred} from './Deferred.js'; // See https://gist.github.com/odbol/1a98cf6186caaace78cae9e7a249c992

/**
 * A PendingRequest.
 */
export interface PendingRequest<T> {
  /**
   * Id of the request for resolving/rejecting later.
   */
  requestId: number;

  /**
   * The Promise that will be resolved/rejected later.
   */
  promise: Promise<T>;
}

/**
 * Keeps track of Promises that are assigned to requestIds, so you can resolve/reject the Promise later according to its
 * requestId.
 *
 * This is useful for providing Promise-based APIs for things that cannot be tracked within a single Promise's callback,
 * e.g. when message passing between frames or ServiceWorkers.
 */
export class PendingRequests<T> {
  private curRequestId = 0;
  private pendingPromises: { [requestId: number]: Deferred<T> }  = {};

  /**
   * Create a new PendingRequest.
   */
  createRequest(): PendingRequest<T> {
    const requestId = this.curRequestId++;
    this.pendingPromises[requestId] = new Deferred();

    return {
      requestId,
      promise: this.pendingPromises[requestId].promise
    }
  }

  /**
   * Resolve an earlier PendingRequest's promise.
   *
   * Returns false if the requestId cannot be found or was already resolved.
   */
  resolveRequest(requestId: number, result: T): boolean {
    const deferred = this.pendingPromises[requestId];
    if (!deferred) {
      console.error(`Invalid request ${requestId}`);
      return false;
    }

    deferred.resolve(result);
    delete this.pendingPromises[requestId];

    return true;
  }


  /**
   * Reject an earlier PendingRequest's promise.
   *
   * Returns false if the requestId cannot be found or was already resolved.
   */
  rejectRequest(requestId: number, error: Error): boolean {
    const deferred = this.pendingPromises[requestId];
    if (!deferred) {
      console.error(`Invalid request ${requestId}`);
      return false;
    }

    deferred.reject(error);
    delete this.pendingPromises[requestId];

    return true;
  }
}