adding promises to an array of promises in a for loop [duplicate]

Question

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JavaScript closure inside loops – simple practical example (44 answers)

Closed 8 years ago.

let's assume the following example:

var bb = require('bluebird');

var promiseStack = [];
var deferred = bb.defer();

promiseStack.push(deferred.promise);

bb.delay(2500).then(function() {
    deferred.resolve();
});

bb.all(promiseStack).then(function() {
    console.log('done');
});

Why isn't it possible to do the following:

var bb = require('bluebird');

var promiseStack = [];


for(var i = 1; i < 10; i++) {
    var deferred = bb.defer();
    promiseStack.push(deferred.promise);

    bb.delay(2500).then(function() {
        deferred.resolve();
    });
}

bb.all(promiseStack).then(function() {
    console.log('done');
});

It takes aprox. 2500ms but console.log('done') isn't called. What's the problem with, am I doing wrong?

The best, redshark1802

Answer 1

What's the problem with, am I doing wrong?

Your deferred variable is not local to the loop body, but on a global scope. You're overwriting it each time with a new Deferred, and resolving only the last of them (but multiple times).

To fix it, you could try a closure, but you shouldn't use Deferred anyway. Just use the promise you already have!

var bb = require('bluebird');

var promiseStack = [];

for(var i = 1; i < 10; i++) // 1 to 9 ???
    promiseStack.push( bb.delay(2500) );

bb.all(promiseStack).then(function() {
    console.log('done');
});