Where to get the callback from the returned promise?

Question

JavaScript

Many times I tried to read all kinds of implementations, but they are still very difficult to understand.

I implement my version of promises, the idea is simple:
After executing the executor function, a callback is triggered that executes resolve().

The then method works like this
1) if the promise is resolved, then the callback inside then is executed immediately;
2) if the promise is not resolved, then the callback is saved, an empty promise is returned. When the executor is executed, it will run the saved callback.

Question - sometimes the content of then gives a promise. How to track the fulfillment of this promise in order to resolve the previous promise? Where to get callback?

'use strict'

function MyPromise (fn) {
  this.executor_link = fn;


  this.state = 'pending';
  this.value = null

  this.reject = function(error) {
    this.value = error;
    this.state = 'rejected';
    if (this.saved_promise) {
      this.saved_err_callback();
      this.saved_promise.reject();
    }
  }

  this.resolve = function (val) {
    this.value = val;
    this.state = 'resolved';
    if (this.saved_promise) {
      let got_promise = this.saved_ok_callback(this.value);
      

      if (got_promise) {
        console.log(got_promise); // откуда брать коллбек из этого промиса? Чтобы потом зарезолвить как в строке ниже
      } else {
        this.saved_promise.resolve(this.value);
      }
  }
}
  

  this.then = function (ok_callback, err_callback) {
    if (this.state == 'pending') {
      this.saved_promise = new MyPromise(() => {});
      this.saved_ok_callback = ok_callback;
      this.saved_err_callback = err_callback;
      return this.saved_promise

    } else if (this.state == 'rejected') {
      return new MyPromise(() => {
        err_callback();
        this.reject();
      })
    } else {
      return new MyPromise(() => {

        ok_callback(this.value);
        
      });
  
    }
  }

  let megares = this.resolve.bind(this);
  let megarej = this.reject.bind(this);

  try { 
  fn(megares, megarej)
  } catch (catched_error) {
    this.reject(catched_error)
  }
}

Answer 1

If I understand your question and your code correctly, then I think you need to do something like this:

if (this.value instansOf MyPromise) {
    return this.value.then(ok_callback, err_callback)
}