Sinon to unit test 'catch' statement - node.js

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)
});
}