One of the most terrible thing about Node.js is that almost every function is asynchronous.

Unlike synchronous programming where you have results returned right from a function call, in asynchronous programming, you need to deal with a lot of callbacks.

Fortunately, async library comes to the rescue.

Let’s look at an example of a highly nested operation where you need to read some file, make a directory, then write some file.

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fs.readFile('somefile.txt', function(err, data) {
  // Use data and do something..
  fs.mkDir('somedirectory', function(err) {
      fs.writeFile('anotherfile.txt', 'some stuff to write', function(err) {
          // Done!
      });
  });
});

Okay, just 3 nests. But I hope you understand how terrible asynchronous programming is.

With async library, you can improve your code. The example above actually requires a waterfall flow, in which each function pass some data to the next function in a sequential way.

This is how you write with async:

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async.waterfall([

  // Task 1
  function(callback) {
      fs.readFile('somefile.txt', function(err, data) {
          callback(null, data);
      }
  },

  // Task 2
  function(data, callback) {
      // Use data and do something..
      fs.mkDir('somedirectory', function(err) {
          callback(null);
      }
  },

  // Task 3
  function(callback) {
      fs.writeFile('anotherfile.txt', 'some stuff to write', function(err) {
          callback(null);
      }
  }

], function (err) {
  // Done!
  // Or if any error encountered along the way!
});

While at first glance, you will think why write more code with async!?

While there are more code, but it is neater code.

Imagine if you have 10 tasks instead of 3 tasks. Using async will be much much neater. What’s more, there are more control flows that async provides.

If you program node.js, I suggest you get started with async library as early as possible.