CODEWITHSUNDEEP
javascriptunit-testingpromisemocha.jschai
Contentop
Contentop

Reputation: 1241

Test with mocha and chai to see if a promise is resolved or rejected

I wrote a function that takes some arguments and inside the function it uses a class that has a function from a 3rd party library that returns a promise and I return this result of the promise in my own function. like :

return client.track(data) // client.track is from a 3rd party lib that should return a promise

Now I want to test the return of the promise in my mocha + chai testing file. I import this function with the inputs and I should get the result. something like:

const result = await myfunc(x,y) // Should return the result of a promise

now I want to test if the promise was actually resolved or rejected in mocha + chai, like using

chai.expect(...)

How can I do this?

Upvotes: 0

Views: 1706

Answers (4)

anangm182
anangm182

Reputation: 36

Maybe you need a specific assertion inside the catch section, something like:

chai.expect(e).to.be.an.instanceOf(Error);
chai.expect(e.message).to.be.not.null;

The expect method comes from the chai library which usually partners with mocha.

Upvotes: 0

Tom Wilson
Tom Wilson

Reputation: 837

Here's what I am using with mocha to test rejected promises. It is ugly and verbose, but avoids adding more test dependencies, and avoids false positives.

describe('when an awaited promise is rejected', () => {
  it('should raise an error', async () => {
    try {
      await someAsyncFunctionThatWillReject()
      assert(false) // will fail the test if no error
    } catch(e) {
      assert(true)  // passes only if there was an error
    }
  })
})

The final assert(true) does not really affect test results, but it makes the test more readable.

Upvotes: 0

Qwerty
Qwerty

Reputation: 31909

This works, but requires a little bit of boilerplate.

await expect(await myPromise.then(() => true)).to.be.true

Upvotes: 0

Keegan Ferrett
Keegan Ferrett

Reputation: 374

Take a look at the npm package chai-as-promised(https://www.npmjs.com/package/chai-as-promised). It's an extension to the chai library which offers support for promises.

You can set it up in the following way

const chai            = require("chai")
const chaiAsPromised  = require("chai-as-promised")

chai.use(chaiAsPromised)
chai.should()

promiseFunction(args).should.be.rejectedWith(SomeError)
promiseFunction().should.eventually.be.a('array')

Take a look at the documentation to get a better idea of the power of this plugin :)

Upvotes: 2

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