Question

141

What is the point of the done() callback?

rated 0 times [ 142] [ 1] / answers: 1 / hits: 46601 / 8 Years ago, sun, june 5, 2016, 12:00:00

In Mochajs, they use done() to test for asynchronous code, like so:

describe('User', function() {

  describe('#save()', function() {

    it('should save without error', function(done) {

      var user = new User('Luna');

      user.save(function(err) {

        if (err) throw err;

        done();

      });

    });

  });

});

What does this mean exactly? I did console.log(done.toString()) and I got this:

function (err) {

  if (err instanceof Error || toString.call(err) === '[object Error]') {

    return done(err);

  }

  if (err) {

    if (Object.prototype.toString.call(err) === '[object Object]') {

      return done(new Error('done() invoked with non-Error: '

        + JSON.stringify(err)));

    }

    return done(new Error('done() invoked with non-Error: ' + err));

  }

  done();

}

Is the done() at the very end here different than the done() in the first piece of code?

Answers

Only authorized users can answer the question. Please sign in first, or register a free account.

havenbilliec

Add To Favorites

Follow

Total Points: 324

Total Questions: 106

Total Answers: 94

Location: Pitcairn Islands

Member since Fri, Oct 15, 2021

3 Years ago

havenbilliec questions

1 Uncaught ReferenceError: process is not defined / Line 0: Parsing error

Thu, Mar 3, 22, 00:00, 2 Years ago

1 React Big Calendar - Is it possible to click on a day and retrieve a list of events on that day?

Fri, Feb 11, 22, 00:00, 2 Years ago

1 Warning: A component is changing a controlled input to be uncontrolled in React js

Fri, May 7, 21, 00:00, 3 Years ago

1 Error: Cannot find module nuxt.js, why my app don't start?

Wed, Apr 7, 21, 00:00, 3 Years ago

1 Redirecting to another html page after successful login attempt

Wed, Nov 25, 20, 00:00, 4 Years ago

View All

answered 8 Years ago jacklynr · Accepted Answer

Mocha is able to handle synchronous and asynchronous tests. When you run a synchronous test, you can just pass it as an anonymous function to it and you don't have to do anything else: Mocha knows the test is over when the function returns. However, if you are running an asynchronous test, you have to tell Mocha that the test is asynchronous. There are two ways to do this:

Declare that the anonymous function you pass to it takes a parameter. Mocha will call your anonymous function with a single parameter which is a function you must call to indicate that your test is over. (This parameter is called done due to tradition. You could call it complete, cb or platypus and it would work just the same.) If you call done without a value, the test is successful. With a value, the test is a failure and the value should be an Error object or an object derived from Error.

Return a promise: Mocha will wait for the promise to be resolved or rejected. If resolved, the test is successful. If rejected, the test failed.

The code you see when you do done.toString() is just the code of the function that Mocha passes to your test when you declare it to take a parameter. You can see in it some of what I mentioned above (e.g. if you pass a parameter to done it should be an Error or derived from Error). The done in there is another done function which is private to Mocha.