testing a function inside another function using jest

Question

how can i test the below snippet using jest. I am trying to test the winston custom format the printf

// sample.js

import {aa:{b}} = require("thirparty-package")

const a = () => {
   return b((log) => {
     return `log message will be ${log.message}`
   })
}

module.exports = {
  a
}


// sample.test.js
const customFunctions = require('./sample')

test('should check b function is called and returns a string', () => {
   expect(customFunctions.a).toHaveBeenCalled() // throwing error 
    //jest.fn() value must be a mock function or spy.
})

It's unclear what's going on. Why should a be called? You're not calling it. — Estus Flask
– Estus Flask, Commented Feb 2, 2019 at 10:33
then how can i know that b will be called. Check the orginal snippet it will give you better view github.com/winstonjs/winston#formats. here if we need a customFormat we can give that but customFormat is not a function it is just a variable. Since i was not able to test it i moved out the customFormat and made it as a functon tried to test it. But it is not working — Beginner
– Beginner, Commented Feb 2, 2019 at 10:37

Estus Flask · Accepted Answer · 2019-02-07 10:39:27Z

1

If it's b that needs to be tested then it should be a spy, not a.

Third-party module should be mocked (a demo):

const bMock = jest.fn();
jest.mock('thirparty-package', () => ({ aa: { b: bMock } }));
const { a } = require('./sample');
a();
const callback = bMock.mock.calls[0][0]; 
expect(callback).toEqual(expect.any(Function));
expect(callback({ message: 'foo' })).toBe('log message will be foo');

edited Feb 7, 2019 at 10:39

answered Feb 2, 2019 at 10:41

Estus Flask

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12 Comments

Beginner Over a year ago

i have already mocked the entire winston library with jest.genMockFromModule('winston');

Beginner Over a year ago

i tried the code but it is not covering the line, still the lines are uncovered

Estus Flask Over a year ago

I don't think that genMockFromModule will generate nested aa object.. You may also need to test a callback, like const cb = bMock.calls[0][1]; expect(cb({ message: ... })).toBe(...);

Beginner Over a year ago

i didn't understand can you update the answer snippet

Beginner Over a year ago

i have posted the entire code , can you check this one stackoverflow.com/questions/54502290/… and suggest a solution

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