Sailcomp Sailcomp - 3 months ago 14
Javascript Question

One Promise for Multiple Promises - Concurrency Issue

In the method "myMethod" of my "gulpfile.js" I want to create multiple Promises. The quantity depends on the array size (method parameter). When I call the method I want to make sure that all promises are fulfilled before I continue. I would prefer to return not before all promises are fulfilled.

Please have a look at the last five lines of code.

Dependencies

var promiseAll = require('gulp-all');
var del = require('del');
var deleteEmpty = require('delete-empty');


gulp-all | del | delete-empty

Helper Method

var oneForAllPromises = function(promises){
var promAll = promiseAll(promises);
promAll.then(function(param) {
console.log('foo');
}, function(err) {
console.error('foo');
});
return promAll;
}


Problematic Code

var myMethod = function(array1, array2){
var promise = del(array1, {force: true});
promise.then(paths => {console.log('foo');});

var promises = [];
promise.then(()=>{
for(var i=0; i<array2.length; i++){
promises[i] = new Promise(function(resolve, reject) {
deleteEmpty(array2[i], {force: true},
function(err, deleted){
if(err){
console.log('foo');
reject
}else{
console.log('foo');
resolve
}
}
);
});
}
});

// PROBLEM: Returns empty promises array
console.log("promiesesLENGTH: "+promises.length); // promiesesLENGTH: 0

// Create one promise for all the promises
return oneForAllPromises(promises);
}

Answer

At the time of the console.log, the first promise promise = del(array1, {force: true}); is not yet finished, so none of the code in the then is yet executed. That's why your promises are empty.

You can simply return in a then another promise:

var myMethod = function(array1, array2){
    var promise = del(array1, {force: true});

    return promise.then(() => {
        return Promise.all(array2.map(array2value => {
            return new Promise(function(resolve, reject) {
                deleteEmpty(array2value, {force: true}, (err, deleted) => {
                    if (err) {
                        reject(err);
                    } else{
                        resolve()
                    }
                });
            });
        }
    });
}
Comments