Simplest way to wait some asynchronous tasks complete, in Javascript?

Question

I want to drop some mongodb collections, but that's an asynchronous task. The code will be:

var mongoose = require('mongoose');  mongoose.connect('mongo://localhost/xxx');  var conn = mongoose.connection;  ['aaa','bbb','ccc'].forEach(function(name){     conn.collection(name).drop(function(err) {         console.log('dropped');     }); }); console.log('all dropped'); 

The console displays:

all dropped dropped dropped dropped 

What is the simplest way to make sure all dropped will be printed after all collections has been dropped? Any 3rd-party can be used to simplify the code.

Answer 1

Use Promises.

var mongoose = require('mongoose');  mongoose.connect('your MongoDB connection string'); var conn = mongoose.connection;  var promises = ['aaa', 'bbb', 'ccc'].map(function(name) {   return new Promise(function(resolve, reject) {     var collection = conn.collection(name);     collection.drop(function(err) {       if (err) { return reject(err); }       console.log('dropped ' + name);       resolve();     });   }); });  Promise.all(promises) .then(function() { console.log('all dropped)'); }) .catch(console.error); 

This drops each collection, printing “dropped” after each one, and then prints “all dropped” when complete. If an error occurs, it is displayed to stderr.

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Previous answer (this pre-dates Node’s native support for Promises):

Use Q promises or Bluebird promises.

With Q:

var Q = require('q'); var mongoose = require('mongoose');  mongoose.connect('your MongoDB connection string'); var conn = mongoose.connection;  var promises = ['aaa','bbb','ccc'].map(function(name){     var collection = conn.collection(name);     return Q.ninvoke(collection, 'drop')       .then(function() { console.log('dropped ' + name); }); });  Q.all(promises) .then(function() { console.log('all dropped'); }) .fail(console.error); 

With Bluebird:

var Promise = require('bluebird'); var mongoose = Promise.promisifyAll(require('mongoose'));  mongoose.connect('your MongoDB connection string'); var conn = mongoose.connection;  var promises = ['aaa', 'bbb', 'ccc'].map(function(name) {   return conn.collection(name).dropAsync().then(function() {     console.log('dropped ' + name);   }); });  Promise.all(promises) .then(function() { console.log('all dropped'); }) .error(console.error);