node.js - Verify that an exception is thrown using Mocha / Chai and async/await

Question

I'm struggling to work out the best way to verify that a promise is rejected in a Mocha test while using async/await.

Here's an example that works, but I dislike that should.be.rejectedWith returns a promise that needs to be returned from the test function to be evaluated properly. Using async/await removes this requirement for testing values (as I do for the result of wins() below), and I feel that it is likely that I will forget the return statement at some point, in which case the test will always pass.

// Always succeeds
function wins() {
  return new Promise(function(resolve, reject) {
    resolve('Winner');
  });
}

// Always fails with an error
function fails() {
  return new Promise(function(resolve, reject) {
    reject('Contrived Error');
  });
}

it('throws an error', async () => {
  let r = await wins();
  r.should.equal('Winner');

  return fails().should.be.rejectedWith('Contrived Error');
});

It feels like it should be possible to use the fact that async/await translates rejections to exceptions and combine that with Chai's should.throw, but I haven't been able to determine the correct syntax.

Ideally this would work, but does not seem to:

it('throws an error', async () => {
  let r = await wins();
  r.should.equal('Winner');

  (await fails()).should.throw(Error);
});

Answer 1

The problem with this approach is that (await fails()).should.throw(Error) doesn't make sense.

await resolves a Promise. If the Promise rejects, it throws the rejected value.

So (await fails()).should.throw(Error) can never work: if fails() rejects, an error is thrown, and .should.throw(Error) is never executed.

The most idiomatic option you have is to use Chai's rejectedWith property, as you have shown in your question.

Here's a quick example. Not much is different from what you've demonstrated in your question; I'm just using async functions for wins() and fails() and expect instead of should. Of course, you can use functions that return a Promise and chai.should just fine.

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

// Always succeeds
async function wins() {
  return 'Winner'
}

// Always fails with an error
async function fails() {
  throw new Error('Contrived Error')
}

it('wins() returns Winner', async () => {
  expect(await wins()).to.equal('Winner')
})

it('fails() throws Error', async () => {
  await expect(fails()).to.be.rejectedWith(Error)
})

If you like want your wins() test to resemble your fails() test more closely, you can write your wins() test like so:

it('wins() returns Winner', async () => {
  await expect(wins()).to.eventually.equal('Winner')
})

The key thing to remember in either of these examples is that chai-as-promised returns promises for its functions such as rejectedWith and eventually.something. Therefore you must await them in the context of an async test function, or else failing conditions will still pass:

async function wins() {
  return 'Loser'
}

async function fails() {
  return 'Winner'
}

it('wins() returns Winner', async () => {
  expect(wins()).to.eventually.equal('Winner')
})

it('fails() throws Error', async () => {
  expect(fails()).to.be.rejectedWith(Error)
})

If you ran the tests with the code above, you'd get the following:

$ npm test

> [email protected] test /home/adaline/code/mocha-chai-async
> mocha .



  √ wins() returns Winner
(node:13836) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rej
ection id: 1): AssertionError: expected 'Loser' to equal 'Winner'
(node:13836) [DEP0018] DeprecationWarning: Unhandled promise rejections are dep
recated. In the future, promise rejections that are not handled will terminate
the Node.js process with a non-zero exit code.
  √ fails() throws Error
(node:13836) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rej
ection id: 2): AssertionError: expected promise to be rejected with 'Error' but
 it was fulfilled with 'Winner'

  2 passing (11ms)

As you can see, the chai assertions actually failed, but they were failed in the context of a Promise that no one ever awaited or catched. So Mocha sees no failure and marks the tests as though they passed, but Node.js (in behaviour that will change in the future as noted above) prints the unhandled rejections to the terminal.