Doing asynchronous tasks serially using ‘async’ in node.js

In the open-event-webapp generator we need to perform a lot of asynchronous tasks in the background like –

  • Downloading images and audio assets
  • Downloading the jsons from the endpoints
  • Generating the html from handelbar templates
  • and so on . .

Sometimes tasks depend on previous tasks, and in such cases we need to perform them serially. Also there are tasks like image downloads, that would be better if done parallelly.

To achieve both these purposes, there is an awesome node.js library called async that helps achieve this.

To perform asynchronous tasks serially (one task, then another task), we can use something like this –

 

async.series([
    (done) => {
       someAsyncFunction(function () { done () })
    },
    //(done) => {..}, (done) => {..} more tasks here
    (done) => {
       someAsyncFunction(function () { done () })
    });
      
    }
]);

Basically async takes an array of functions. Each function contains a callback that you need to call when the internal task is finished. The 2nd task starts, only after the done() callback of first task is executed.

An example of it’s usage can be seen in the open-event-webapp project here