Sinon to unit test 'catch' statement

Question

I've got a simple function such as;

module.exports = {
  fetchUser:function(myUserId) {
    return new Promise((resolve, reject) => {
      this.getUser(myUserId)
        .then(user => {
          // do logic // then return user

          return user;
        })
        .then(resolve)
        .catch(err => {
          // whoops there has been an error

          let error = { error: 'My Error' };

          reject(error);
        });
    });
  }
};

I want to unit test both the resolve and reject result.

A simple chai test would be;

var expect = require('chai').expect;
var user = require('./user');

describe('User module', function() {
  it('test fetchUser', function() {
    let _user = user.fetchUser('abc123');

    return _user
      .then(user => {
        expect(data).to.be.an('object');
      });
  });

Using sinon or another library, how can I for the fetchUser function to throw that reject error?

Answer 1

With Mocha, Chai and Sinon it can be implemented with stubbed method getUser.

const User = require("./fetchUserModule");

describe('User module', () => {
    beforeEach(() => User.getUser = sinon.stub());

    afterEach(() => User.getUser.reset());

    it('returns user if `getUser` returns data', () => {
        const user = {name: 'John'};
        User.getUser.withArgs("abc123").returns(Promise.resolve(user));

        return User.fetchUser("abc123").then(result => {
            expect(result).to.equal(user)
        }).catch(error => {
            expect(error).to.be.undefined;
        })
    });

    it('throws error if `getUser` is rejected', () => {
        User.getUser.withArgs("abc123").returns(Promise.reject());

        return User.fetchUser("abc123").then(result => {
            expect(result).to.be.undefined;
        }).catch(err => {
            expect(err).to.eql({error: 'My Error'})
        })
    });
});

Answer 2

Start with anything in your "logic" that can throw an error.

If not you would need to stub this.getUser to reject or throw an error instead of returning data. sinon-as-promised patches sinon.stub to include the .resolves and .rejects promise helpers.

const sinon  = require('sinon')
require('sinon-as-promised')

Setup the stub for the failure tests.

before(function(){
  sinon.stub(user, 'getUser').rejects(new Error('whatever'))
})

after(function(){
  user.getUser.restore()
})

Then either catch the .fetchUser error or use chai-as-promised for some sugar.

it('test fetchUser', function() {
  return user.fetchUser('abc123')
    .then(()=> expect.fail('fetchUser should be rejected'))
    .catch(err => {
      expect(err.message).to.eql('whatever')
    })
})

it('test fetchUser', function() {
  return expect(user.fetchUser('abc123')).to.be.rejectedWith(Error)
})

or async if you live in the new world

it('test fetchUser', async function() {
  try { 
    await user.fetchUser('abc123')
    expect.fail('fetchUser should be rejected'))
  } catch(err) {
    expect(err.message).to.eql('whatever')
  }
})

As a side note, you don't need to wrap something that already returns a promise in new Promise and be careful about losing error information when chaining multiple .catch handlers.

fetchUser: function (myUserId) {
  return this.getUser(myUserId)
    .then(user => {
      //logic
      return user
    })
    .catch(err => {
      let error = new Error('My Error')
      error.original = err
      reject(error)
    });
}