Mocking a promise returning method from the same class

Question

I want to mock/stub the resolve of a class method that returns a promise when testing another method of the same class. I'm using jest.

I have the following class Blah of which I want to test the foo() method:

export default class Blah {
  foo () {
    this.bar().then(result => {
      // some logic here
    })
  }

  bar () {
    return new Promise(function (resolve, reject) {
      // fetches some stuff over the network
      resolve('hello world')
    })
  }
}

Because foo() uses bar() - a promise returning method that does stuff over the network - I would like to mock the resolve of bar(); let's say make it resolve what up.

Let's say the test would look something like this:

import Blah from './blah'

test('foo() - bar() resolves "what up"', () => {
  const blah = new Blah()
  // mock blah.bar() so that is resolves "what up"

  // {some assertions on blah.foo() here}
})

It's my first time working with jest and I've read through the docs, but I'm still having a hard time wrapping my head around this case.

How can I mock the resolved value of bar() while testing foo()?

Jared Smith · Accepted Answer · 2018-05-30 17:31:33Z

1

Still just JavaScript meaning any object property can be dynamically overwritten:

// in the test file
const blah = new Blah();
blah.bar = _ => Promise.resolve("what up");

// now some assertions about blah.foo

answered May 30, 2018 at 17:31

Jared Smith

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2 Comments

Juicy Over a year ago

thanks, if you'll pardon my ignorance, why the _ in this context?

Jared Smith Over a year ago

@Juicy it's a fairly common convention for "I don't care what arguments are passed to this function". The underscore (and dollar sign) are valid JavaScript identifiers. I could have written blah.bar = dontcare => Promise.resolve... just force of habit :)

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