Unit-testing vanilla JS with hard-coded dependencies

Question

Imagine I have a legacy JS app with code like this:

var thing = new Thing();
function do_thing() {
  // Logic to test here
}
// export module

This file also includes a number of important calculations I need to unit-test, without refactoring the whole thing.

I tried to test it with jest.

/**
 * @jest-environment jsdom
 */
const mod = require('thing.js');

test('Test', () => {
  mod.do_thing();
});

but get

ReferenceError: Thing is not defined

Even removing var thing will cause errors if do_thing has calls to functions defined in other files, as is usually the case in legacy projects.

ReferenceError: foo() is not defined

Any tips? Possible to mock out things using jest.fn or jest.mock similar without heavy refactoring of the app?

Answer 1

Solution using global.Thing:

/**
 * @jest-environment jsdom
 */
global.Thing = jest.fn();
const mod = require('thing.js');

test('Test', () => {
  mod.do_thing();
});