How do I fail a Node unit test on the catch of a Promise?

Question

I'm doing some unit tests using Node.js and I want to fail a test like this:

doSomething()
    .then(...)
    .catch(ex => {
        // I want to make sure the test fails here
    });

I'm using Assert, so I found Assert.Fails . The problem is that fails expects actual and expected , which I don't have. The Node documentation doesn't say anything about them being required, but the Chai documentation , which is Node compliant, say they are.

How should I fail a test on the catch of a promise?

Answer 1

You can use a dedicated spies library, like Sinon , or you can implement a simple spy yourself.

function Spy(f) {
  const self = function() {
    self.called = true;
  };
  self.called = false;
  return self;
}

The spy is just a wrapper function which records data about how the function has been called.

const catchHandler = ex => ...;
const catchSpy = Spy(catchHandler);

doSomething()
  .then(...)
  .catch(catchSpy)
  .finally(() => {
    assert.ok(catchSpy.called === false);
  });

The basic principle is to spy on the catch callback, then use the finally clause of your promise to make sure that the spy hasn't been called.

Answer 2

Have you considered Assert.Ok(false, message)? It's more terse.

Assert.fail is looking to do a comparison and display additional info.

Answer 3

If you will use mocha , then elegant way would be as following:

describe('Test', () => {
  it('first', (done) => {
    doSomething()
    .then(...)
    .catch(done) 
    })
})

If your Promise fail, done method will be invoked with the thrown exception as a parameter, so code above is equivalent to

catch(ex => done(ex))

In mocha invoking done() with the parameter fails the test.