Test a rejection with Chai as promised

javascript chai
Guid picture Guid · Mar 31, 2015 · Viewed 29k times · Source

I want to test a function returning a promise.

In this particular test, the promise is expected to be rejected with an Error object containing the classical message field (in this test, it is expected to equal "my error message") and a custom field I added named code, which is a string (like "EACCESS", "ERIGHT", etc, in this test it is expected to equal "EFOO")

I want to use chai-as-promised for that.

return expect(foo()).to.eventually.be.rejectedWith("my error message");

This assertion is working but now I would like to test the code field too.
How to do that?

Answer

Keithamus picture Keithamus · Sep 25, 2015

If you're using Chai-As-Promised (as you say you are), then it allows for chaining off of rejectedWith - and it sets the chain assertion object to be the error object - meaning anything after rejectedWith() is now going to assert on the Error. This lets you do cool things like:

return expect(foo()).to.eventually
  .be.rejectedWith("my error message")
  .and.be.an.instanceOf(Error)
  .and.have.property('code', 'EFOO');

Some of the chai methods also chain, so you can use that to make some quite deeply nested assertions about the error:

return expect(foo()).to.eventually
  .be.rejectedWith("my error message")
  .and.have.property('stack')
    .that.includes('myfile.js:30')

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