Deferred: a Promise that you can resolve or reject after the fact.

Deferred.ts

/**
 * A Promise that you can resolve or reject after the fact.
 */
export class Deferred<T> {
  private _resolve?: (result: T) => void;
  private _reject?: (error: Error) => void;
  promise: Promise<T>;

  constructor() {
    this.promise = new Promise((resolve, reject) => {
        this._resolve = resolve;
        this._reject = reject;
    });

    this.reject = this.reject.bind(this);
    this.resolve = this.resolve.bind(this);
  }

  resolve(result: T) {
    this._resolve!!(result);
  }

  reject(error: Error) {
    this._reject!!(error);
  }
}