"Done" function in javascript

Question

For example, I read Grunt documentation on Creating Tasks. In "Tasks can be asynchronous" section, there is a use case of "done" function, I wonder why do we need that.

grunt.registerTask('asyncfoo', 'My "asyncfoo" task.', function() {
  // Force task into async mode and grab a handle to the "done" function.
  var done = this.async();
  // Run some sync stuff.
  grunt.log.writeln('Processing task...');
  // And some async stuff.
  setTimeout(function() {
    grunt.log.writeln('All done!');
    done();
  }, 1000);
});

It's a callback to say the task is done, if a task is async it needs the callback to know when it's done (and if is fails or not) — michelem, Jul 24 '15 at 07:22

score 3 · Accepted Answer · edited May 23 '17 at 12:02

It's the only way for Grunt to know when your task is complete when doing async work.

This SO Post provides quite a bit of detail of what async in JS means and how to work with it.

Basically, setTimeout calls your callback function 1 second later. However, grunt has no idea when your callback is complete, as setTimeout itself returns immediately once it is called. Therefore, the done() function exists to provide you a way to tell grunt that your task is complete, whenever that may be.

score 0 · Answer 2 · answered Jul 24 '15 at 07:23

Grunt internally has a flag that determines if a task should be marked as complete when the task-function finishes or if it should be handled asynchronously. By default Grunt runs tasks synchronously and the this.async() call toggles this flag.

The "synchronous-by-default" design decision was probably made to make simple tasks easier to write where you could use fs.writeSync etc., which is definitely a valid approach albeit gets in the way with at least the kind of tasks that I usually write nowadays.

Grunt was also created before Promises became a viable solution for callback-hells, so there's that too.

I'm pretty sure there are also other reasons for this that I'm not aware of :)

"Done" function in javascript

2 Answers2