Jest.js - test that specific function is called by the module

Question

The sole purpose of index.js is to run foo and bar functions:

//index.js
import foo from './foo.js'
import bar from './bar.js'

foo()
bar()

How can I test that when running index.js both foo and bar get executed?

Answer 1

Firstly I would concentrate on testing foo and bar functions making sure they do what they have to do.

For example if the function foo is ()=>{ return 'foo() is working' } I make a file called foo.test.js and i write 2-3 tests for it.

test('make sure it returns something', () => {
  expect(!foo()).toBe(false);
});
test('make sure it returns string', () => {
  var val = foo();
  expect(typeof val === 'string').toBe(true);
});
test('make sure it returns string', () => {
  expect(foo()==='foo() is working').toBe(true);
});

This will make sure my foo.js is doing what it suppose to then I make another file called index.text.js and in it I'll do my mock test

jest.mock("../foo");
const foo = require("../foo");

test('make sure foo is called', () => {
  expect(foo).toBeCalled();
});
test('make sure foo run only once', () => {
   expect(foo.mock.calls.length).toBe(1);
});

Now assuming you have installed jest and have package.json configured correctly you can simply run : npm run test or if you have yarn you can just run yarn test...