CODEWITHSUNDEEP
javascriptpromiseecmascript-6es6-promise
Michael Plakhov
Michael Plakhov

Reputation: 2291

What is the reason to allow multiple resolve/reject in ES6 Promise

I found that is possible (in ES6 promises, while Promise object is created) to use multiple resolve/reject which will affect PromiseStatus only once but not affect execution flow. 

var p = new Promise(function(resolve, reject) { 
    setTimeout(function(){
        resolve(1);
        console.log('Resolve 1');
    }, 50);
    setTimeout(function(){
        resolve(2);
        console.log('Resolve 2');
    }, 100);
});

setTimeout(function(){
        console.log('Status #1:', p);
    }, 10);
setTimeout(function(){
        console.log('Status #2:', p);
    }, 60);
setTimeout(function(){
        console.log('Status #3:', p);
    }, 110);

p.then(function(x){
    console.log('Value after:', x)
})

In then() functions first resolve/reject will affect execution flow. So my question is - why it works like this (feature/bug)?

P.S. My env is Node 4.1

P.P.S. My output:

Status #1: Promise { <pending> }
Resolve 1
Value after: 1
Status #2: Promise { 1 }
Resolve 2
Status #3: Promise { 1 }

Upvotes: 5

Views: 1759

Answers (2)

Benjamin Gruenbaum
Benjamin Gruenbaum

Reputation: 276306

Well, I want to talk about the why. Promises are proxies for single values so running handlers a second time or changing the value doesn't make sense. You can't change the number 5 to be the number 3 for instance.

Let's talk about what alternatives we have for resolve being called a second time. Let's say we didn't want to allow it - how would we signal that?

Normally, we'd throw - the problem is - it would be caught nowhere since throws in the promise constructor get converted to rejections. .catch handlers would not run because the promise already resolved.

So we can't really throw as that'd mean an exception you can't handle (a very bad place to be). We can't run handlers twice (that would break the model). So our only choice left is to allow it.

Upvotes: 8

thefourtheye
thefourtheye

Reputation: 239473

As per the ECMAScript 2015 Specification, sections Promise Reject Functions and Promise Resolve Functions say that,

  1. If alreadyResolved.[[value]] is true, return undefined.

So, if the current promise object is already resolved, then neither resolve, nor reject do anything to the Promise object. It actually means that, only the first resolve/reject matter.

Upvotes: 7

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