what's wrong with my javascript promise code

Question

I want to implement a javascript promise by myself to understand the mechanism, here is my code, but it report undefined error, could someone help me to take a look?

var Promise = function(){
    this.successesCallback = [];
}

Promise.prototype.then = function(success){
    console.log("add success");
    this.successesCallback.push(success);
}

var Defer = function(){
    this.promise = new Promise();
}

Defer.prototype.resolve = function(){
    console.log("defer resolve is calling");
    console.log("2promise of defer:" + this.promise)
    this.promise.successesCallback[0]();
}

var remoteCall = function(callBack){
    for(var i = 0; i < 1000000000; i++){
    }
    callBack();
}

var callRemote = function(){
    var defer = new Defer();
    console.log("promise of defer:" + defer.promise)
    console.log("set timer for remote call");
    setTimeout(function(){remoteCall(defer.resolve)}, 0);
    console.log("remote call is triggered");

    return defer.promise;
}

callRemote().then(function(){console.log("Hello, server call done")});

You can run by node

Add more details related to you problem and things you have already tried to resolve issue. — mitpatoliya
– mitpatoliya, Commented Jan 1, 2016 at 15:22
Wow, a ton of things are wrong in your promise code. For one, your promises don't implement a then correctly since you cannot return a promise from the then, there is no error handling (a catch), and errors in the promise chain cannot be recovered from. — Benjamin Gruenbaum
– Benjamin Gruenbaum, Commented Jan 1, 2016 at 19:46
Have a look at some minimal "promise" libraries linked to here to see how far short your "implementation" of a well known spec really is — Jaromanda X
– Jaromanda X, Commented Jan 2, 2016 at 2:55

Sirko · Accepted Answer · 2016-01-01 15:28:21Z

2

you lose the binding between defer and resolve() in the setTimeout() callback.

One solution would be to use bind():

setTimeout(function(){remoteCall(defer.resolve.bind( defer )}, 0);

answered Jan 1, 2016 at 15:28

Sirko

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2 Comments

Jason Dou Over a year ago

This is it. Thanks so much.

Sirko Over a year ago

@JasonDou If an answer solved your problem, please "accept" it with the checkmark next to it, so others can see, that the problem is solved.

Zoe - Save the data dump · Accepted Answer · 2019-04-29 16:22:36Z

0

You lose the binding between defer and resolve() in the setTimeout() callback, as mentioned in the other answer. Now you can use ()=> instead of function() to maintain the current binding since ES6.

setTimeout(()=>{remoteCall(defer.resolve)}, 0);

edited Apr 29, 2019 at 16:22

Zoe - Save the data dump

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answered Jan 3, 2018 at 23:28

TimCrowe

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