What is the right way to unit test asynchronous functions in NodeJS using Mocha?

Question

I need an extremely simple example on how to test function like this:

// Read from Redis DB
Main.prototype.readFromRedis = function(key, callback){
    dbClient.get(key, function(err, reply) {
        return callback(err, reply)
    });
}

or this:

// Write to file
Main.prototype.writeToFile = function(fileName, content, callback){
    fs.appendFile(fileName, content, encoding='utf8', function (err) {
        return callback(err);
    });
}

I've been searching for the optimal solution for quite a while but wasn't able to find anything very useful.

My attempts:

describe('main', function() {
  it('readFromRedis(key, callback) should read value from DB', function() {
    var main = new Main();
    var error;
    main.readFromRedis('count',function(err){
        err = error;
        });
    expect(error).to.equal(undefined);
  });
});

However here the expect() executes before the readFromRedis besides that I don't find my solution to be the right one.

Have you seen the [async code](https://mochajs.org/#asynchronous-code) section in the mocha docs? You can use `done` in this scenario. — dan, May 30 '17 at 12:05

dan · Accepted Answer · 2017-05-30T12:12:22.097

Mocha has support for aync functions (docs). You can use done to signal the end of the test. Like this:

describe('main', function() {
  // accept done as a parameter in the it callback
  it('readFromRedis(key, callback) should read value from DB', function(done) {
    var main = new Main();
    main.readFromRedis('count',function(err){
        expect(error).to.equal(undefined);
        // call done() when the async function has finished
        done()
    });
  });
});

When you pass done as a parameter into the it callback, then the test won't complete until done has ben called, or the timeout has been reached.

What is the right way to unit test asynchronous functions in NodeJS using Mocha?

1 Answers1