# Synchronous vs Asynchronous programming in nodejs

# Using async

The async package (opens new window) provides functions for asynchronous code.

Using the auto (opens new window) function you can define asynchronous relations between two or more functions:

var async = require('async');

    get_data: function(callback) {
        console.log('in get_data');
        // async code to get some data
        callback(null, 'data', 'converted to array');
    make_folder: function(callback) {
        console.log('in make_folder');
        // async code to create a directory to store a file in
        // this is run at the same time as getting the data
        callback(null, 'folder');
    write_file: ['get_data', 'make_folder', function(results, callback) {
        console.log('in write_file', JSON.stringify(results));
        // once there is some data and the directory exists,
        // write the data to a file in the directory
        callback(null, 'filename');
    email_link: ['write_file', function(results, callback) {
        console.log('in email_link', JSON.stringify(results));
        // once the file is written let's email a link to it...
        // results.write_file contains the filename returned by write_file.
        callback(null, {'file':results.write_file, 'email':'user@example.com'});
}, function(err, results) {
    console.log('err = ', err);
    console.log('results = ', results);

This code could have been made synchronously, by just calling the get_data, make_folder, write_file and email_link in the correct order. Async keeps track of the results for you, and if an error occurred (first parameter of callback unequal to null) it stops the execution of the other functions.