"Done" function in javascript - Stack Overflow

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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);
});

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);
});

• javascript

Share Improve this question Follow edited Jul 24, 2015 at 7:53 philomath asked Jul 24, 2015 at 7:09 philomathphilomath 2,20966 gold badges3434 silver badges4646 bronze badges 1

• 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 Commented Jul 24, 2015 at 7:22

Add a ment  | 

2 Answers 2

Sorted by: Reset to default 3

It's the only way for Grunt to know when your task is plete 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 plete, 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 plete, whenever that may be.

Grunt internally has a flag that determines if a task should be marked as plete 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 :)

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