Testing JS exceptions with Mocha/Chai

Question

Trying to test some code that throws an exception with Mocha/Chai, but having no luck, here's the simple code I'm trying to test:

class window.VisualizationsManager

  test: ->
    throw(new Error 'Oh no')

Here is my test:

describe 'VisualizationsManager', ->

  it 'does not permit the construction of new instances', ->

    manager = new window.VisualizationsManager

    chai.expect(manager.test()).to.throw('Oh no')

However, when I run the spec, the test fails and throws the exception.

Failure/Error: Oh no

what am I doing wrong here?

Solution 1

It's probably because you are executing the function right away, so the test framework cannot handle the error.

Try something like:

chai.expect(manager.test.bind(manager)).to.throw('Oh no')

If you know that you aren't using the this keyword inside the function I guess you could also just pass manager.test without binding it.

Also, your test name doesn't reflect what the code does. If it doesn't permet the construction of new instances, manager = new window.VisualizationsManager should fail.

Solution 2

Either pass the function:

chai.expect(manager.test).to.throw('Oh no');

Or use an anonymous function:

chai.expect(() => manager.test()).to.throw('Oh no');

See the documentation on the throw method to learn more.