Testing Asyc Functions with chai-as-promised

Set up chai-as-promised:

const chai = require('chai');
const chaiAsPromised = require('chai-as-promised');
chai.use(chaiAsPromised);
const expect = chai.expect;

General syntax:

it('Promise should be rejected with CustomError', function () {
  return expect(asyncFn()).to.be.rejectedWith(CustomError);
});

it('Promise should pass', function () {
  return expect(asyncFn()).to.be.fulfilled;
});

// async-await alternative syntax
it('should work well with async/await', async function () {
  await expect(asyncFn()).to.be.rejectedWith(Error);
});