Where to get the callback from the returned promise?

• 0
  I tried to read all kinds of implementations many times, but they are still very difficult to understand.
  
  Implementing my own version of promises, the idea is simple:
  After executing the executor function, a callback is triggered, which executes resolve ().
  
  The then method works like this
  1) if the promise is resolved, then the callback is executed immediately inside then;
  2) if the promise is not resolved, then the callback is saved, an empty promise is given. When the executor is executed, it will run the saved callback.
  
  Question - sometimes the content of then makes a promise. How can I track the execution of this promise in order to catch the previous promise? Where to get the callback from?
  
  

  '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)
    }
  }

  JavaScript Anonymous, Sep 26, 2020

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1 Answers
• 0
  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)
  
  }

  Jack Valentine

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