use promise as a immutable state machine (一种将 Promise 当成状态机使用的实践，爷非常喜欢，只有用过或者遇到过类似逻辑的人才懂)

promise-machine.ts

/** a way to use promise as an immutable state machine */
export class PromiseMachine<T> {
  /** this promise instance */
  private $promise: Promise<T>;
  /** make this resolved */
  private $resolve!: (val: T) => void;
  /** make this rejected */
  private $reject!: (err: any) => void;

  /** 全部が終わった */
  public isFrozen = false;

  /** construing a promise instance for this */
  public constructor() {
    this.$promise = new Promise((resolve, reject) => {
      this.$resolve = resolve;
      this.$reject = reject;
    });
  }

  /** isFrozen -> true (promise resolved) */
  public resolve = (val: T) => {
    if (this.isFrozen) return;
    this.isFrozen = true;
    this.$resolve(val);
  }

  /** isFrozen -> true (promise rejected) */
  public reject = (err: any) => {
    if (this.isFrozen) return;
    this.isFrozen = true;
    this.$reject(err);
  }

  /** map this machine to a promise instance */
  public toPromise = () => this.$promise;
}