Using a Javascript Promise in a for loop

Question

I'm a total n00b to Promises, and would like to use them more, but need a little help. Take the code example below:

for (var key in list){
    DoSomething(list[key])
        .then(function(){
            console.log(key);
        });
}

function DoSomething(item){
    return new Promise(function(resolve, reject){
        resolve();
    });
}

The console.log(key) part wont work correctly because the variable key is modified in the outer scope. A way to fix that might be to resolve the promise with the key value but unfortunately, the DoSomething method is part of a shared library.

for (var key in list){
    DoSomething(list[key], key)
        .then(function(key){
            console.log(key);
        });
}

function DoSomething(item, key){
    return new Promise(function(resolve, reject){
        resolve(key);
    });
}

Is there any other way to get the for loop console.log(key) to work correctly?

using "let" should do the work... – Giorgi Moniava Sep 26 '17 at 13:06 — Giorgi Moniava, Sep 26 '17 at 13:06

score 2 · Accepted Answer · answered Sep 26 '17 at 13:05

2

Use let instead of var and it would work

let list = {a:1, b:2};

for (let key in list){
    DoSomething(list[key])
        .then(function(){
            console.log(key);
        });
}

function DoSomething(item){
    return new Promise(function(resolve, reject){
        resolve();
    });
}

answered Sep 26 '17 at 13:05

marvel308

9,972
1
18
31

Please close-vote obvious duplicates instead of answering them. – Cerbrus Sep 26 '17 at 13:09
@Cerbrus Are you sure you specified correct duplicate? – Giorgi Moniava Sep 26 '17 at 13:13
@GiorgiMoniava: Maybe not the best one, but it does answer the question. – Cerbrus Sep 26 '17 at 13:14
@Cerbrus I fail to see how though – Giorgi Moniava Sep 26 '17 at 13:23
1

@Cerbrus The "Closure inside loop - simple practical example' Q might be better – Andrew Li Sep 26 '17 at 13:56

score 1 · Answer 2 · answered Sep 26 '17 at 13:05

1

Use IIFE

for (var key in list){
    (function(key){
        DoSomething(list[key])
        .then(function(){
            console.log(key);
        });
     })(key);
}

answered Sep 26 '17 at 13:05

Nikhil Aggarwal

27,657
4
40
56

@Downvoter - Reason please – Nikhil Aggarwal Sep 26 '17 at 13:06
Since the OP's using Promises already, ___let___'s suggest some more modern methods than an IIFE. also, please close-vote obvious duplicates instead of answering them. – Cerbrus Sep 26 '17 at 13:07
@Cerbrus - For that I wish there was es6 tag. – Nikhil Aggarwal Sep 26 '17 at 13:08
promises are ES6. – Cerbrus Sep 26 '17 at 13:10
1

ES6 aside, this is a pretty cool way to solve the problem that I didnt know before. Thanks. +1 – Jimbo Sep 26 '17 at 13:12
@Cerbrus - Thanks! Noticed now! You have got the point proved, however, I would still argue on the answer, It is not incorrect and the best part being OP acknowledges it! – Nikhil Aggarwal Sep 26 '17 at 13:13

Using a Javascript Promise in a for loop

2 Answers2