#JavaScript - #Mocha #Chai async await promise test

How to test promises / async await functions using Mocha test runner and Chai ?

Mocha

Chai 

To catch an error from an await function, you can use try and catch blocks like the following:

try {

  await asyncFunction(2);

} catch (err) {

  expect(err).to.be.an.instanceof(Error);

  expect(err.message).to.equal('Service is not available');

}

Testing resolved and rejected promises

Expect resolved promises

// using done callback

it('test resolved promise', (done) => {

  asyncFunction(arguments)

    .then((response) => {

      assert.isString(response);

      expect(response).to.be.a('string');

      expect(response).to.have.lengthOf(2);

      done();

    })

    .catch((err) => {

      done(err);

    });

});

// using async await, so you need to use es2017

it('test resolved promise', async () => {

  const response = await asyncFunction(arguments).catch((err) => { throw err; });

  assert.isString(response);

  expect(response).to.be.a('string');

  expect(response).to.have.lengthOf(2);

});

Expect rejected promises

it('test rejected promise', (done) => {

  asyncFunction(arguments)

    .then(() => {

      assert.fail(0, 1, 'Expected rejected promise'); // to fail a test

      done();

    }, (err) => {

      expect(err).to.be.an.instanceof(Error);

      expect(err.message).to.equal('Service is not available')

      done();

    });

});

it('test rejected promise', async () => {

  const errorHandler = function (err) {

    expect(err).to.be.an.instanceof(Error);

    expect(err.message).to.equal('Service is not available');

  };

  const response = await asyncFunction(arguments).catch(err => errorHandler(err));

  expect(response).to.be.an('undefined');

});

Thanks for reading!

Jun