deferred.ts

/**
 * @license
 * Copyright 2017 Google LLC
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

export class Deferred<R> {
  promise: Promise<R>;
  reject: (value?: unknown) => void = () => {};
  resolve: (value?: unknown) => void = () => {};
  constructor() {
    this.promise = new Promise((resolve, reject) => {
      this.resolve = resolve as (value?: unknown) => void;
      this.reject = reject as (value?: unknown) => void;
    });
  }

  /**
   * Our API internals are not promiseified and cannot because our callback APIs have subtle expectations around
   * invoking promises inline, which Promises are forbidden to do. This method accepts an optional node-style callback
   * and returns a node-style callback which will resolve or reject the Deferred's promise.
   */
  wrapCallback(
    callback?: (error?: unknown, value?: unknown) => void
  ): (error: unknown, value?: unknown) => void {
    return (error, value?) => {
      if (error) {
        this.reject(error);
      } else {
        this.resolve(value);
      }
      if (typeof callback === 'function') {
        // Attaching noop handler just in case developer wasn't expecting
        // promises
        this.promise.catch(() => {});

        // Some of our callbacks don't expect a value and our own tests
        // assert that the parameter length is 1
        if (callback.length === 1) {
          callback(error);
        } else {
          callback(error, value);
        }
      }
    };
  }
}