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

I see you are using mongoose so you are talking about server-side JavaScript. In that case I advice looking at async module and use async.parallel(...). You will find this module really helpful – it was developed to solve the problem you are struggling with. Your code may look like this

var async = require('async');

var calls = [];

['aaa','bbb','ccc'].forEach(function(name){
    calls.push(function(callback) {
        conn.collection(name).drop(function(err) {
            if (err)
                return callback(err);
            console.log('dropped');
            callback(null, name);
        });
    }
)});

async.parallel(calls, function(err, result) {
    /* this code will run after all calls finished the job or
       when any of the calls passes an error */
    if (err)
        return console.log(err);
    console.log(result);
});