扩展`Promise`并改变`then`签名

Question

I want to extend Promise and change the then signature so its callback receives two values. I tried different approaches two of which are documented and tested here . Sadly, I get various errors or the resulting class does not behave like a Promise.

Approach 1: Wrapping a native Promise

export class MyWrappedPromise {
  constructor(data) {
    this.data = data;
    this.promise = new Promise(evaluate.bind(data));
  }

  then(callback) {
    this.promise.then(() => callback(this.data, ADDITIONAL_DATA));
  }

  catch(callback) {
    this.promise.catch(callback);
  }
}

Approach 2: Extending native Promises

export class MyExtendedPromise extends Promise {

  constructor(executor, data) {
    super(executor);
    this.data = data;
  }

  static create(data) {
      return new MyExtendedPromise(evaluate.bind(data), data);
  }

  then(callback) {
    return super.then(() => callback(this.data, ADDITIONAL_DATA));
  }
}

Does anyone have any suggestion on what I am doing wrong? Feel free to create a PR on GitHub.

thanks

------------------- Edit ---------------------

Some Additional code and info to make the code above more understandable without looking at the code and tests on Github.

evaluate is just the Promise executor function. I extracted it out so I can keep it consistent across all my implementations and tests. It may look convoluted but it's structured that way to simulate my "real" project.

export function evaluate(resolve, reject) {
  const data = this;
  function getPromise(data) {
    return !!data ? Promise.resolve(data) : Promise.reject(new Error("Error"));
  }

  getPromise(data)
    .then(resolve)
    .catch(reject);
}

ADDITIONAL_DATA is just a string to simulate the second value in the callback. It's also extracted to be consistent across all versions and tests.

------------------- Edit 2---------------------

Errors that come up depending on the solution

• catch is not accessible
• A lot of UnhandledPromiseRejectionWarning: warnings because errors/rejects are not getting propagated up correctly.
• Errors/rejects are getting thrown too early and don't even reach the rejects checks in my test suites

Answer 1

I don't understand very well why you do have a factory method, instead of using directly the constructor.

Do you mean something like this?

 class MyExtendedPromise extends Promise { constructor(executor, data) { super(executor); this.data = data; } then(callback, test) { console.log('passed new parameter in then:', test); console.log('additional data:', this.data); return super.then(data => callback(data, test)); } } new MyExtendedPromise((resolve, reject) => { setTimeout(() => resolve(true), 2000); }, 'other additional data').then(data => console.log('my then', data), 'hello world'); 

Answer 2

You have problems (especially with unhandled rejections) because you are not implementing the then interface correctly. Remember that .catch(onRejected) is just an alias for .then(undefined, onRejected) , and then with two parameters is the actual core method of every promise .

You were always ignoring the second argument, so no rejection ever got handled. You need to write

then(onFulfilled, onRejected) {
  return super.then(res => onFulfilled(res, this.ADDITIONAL_DATA), onRejected);
  // or `this.promise.then` instead of `super.then`
}