Completer class that allows deferring Promise resolution

completer.js

// Completer
//
// Wrap a Promise and expose `resolve'`/`reject` safely to allow
// resolution/rejection to be determined after promise is created.
//
// Use:
// ```js
// const completer = new Completer();
//
// async function step1() {
//   console.log('doing async work...');
//   // do async work...
//   // after async work, set the completer
//   completer.complete();
//   console.log('done with step1');
// }
//
// async function step2() {
//   console.log('also doing async work...');
//   // do async work here...
//   // Wait for completer since we need step1 to finish first
//   await completer.value;
//   console.log('continuing with more async work...');
//   // do more async work...
//   console.log('done with step2');
// }
//
// async function run() {
//   // Run both promises concurrently...
//   await Promises.all([step1(), step2()]);
//   // Order shouldn't matter, step2 should always wait for step1
//   // await Promises.all([step2(), step1()]);
// }
//
// run();
// ```
class Completer {
  constructor() {
    this.value = new Promise((resolve, reject) => {
      this._resolve = resolve;
      this._reject = reject;
    });
  }

  complete(value) {
    if (this._resolve) {
      this._resolve(value);
      this._resolve = null;
      this._reject = null;
    }
  }

  error(err) {
    if (this._reject) {
      this._reject(err);
      this._resolve = null;
      this._reject = null;
    }
  }
}