How can I mock an ES6 module import using Jest?

Mocking, the proper way

1. Dodge mutation side effects

Directly mutating an import like dependency.doSomething = jest.fn()? Not a good move. It's like asking your tests to throw disco inferno. Keep them chill, use jest.mock instead.

2. Differentiate default and named exports

Here's how you would tackle default exports:

import myDefault from './defaultModule';
jest.mock('./defaultModule', () => ({
  __esModule: true, // ES6 and Jest sitting in the tree...
  default: jest.fn().mockImplementation(() => 'mocked default export'),
}));


Doing some bluffing with named exports? Easy as pie:

import * as myModule from './namedModule';
jest.mock('./namedModule', () => ({
  __esModule: true,
  myNamedExport: jest.fn().mockImplementation(() => 'mocked named export'),
}));


With __esModule: true, Jest gives an ES6-style thumbs-up to your fakery.

3. Sneakily spy without reprogramming

Want spy glasses without the disguise kit? Use jest.spyOn!

import * as myModule from './myModule';
jest.spyOn(myModule, 'myFunction'); // Inspector Gadget activated


4. Tailor mocks with jest.fn()

When every test is a new fashion show, tailor your mock to strut its stuff with jest.fn(). Custom behavior for every test? You got it!

5. Advanced mock tactics

Leveling up? Set up custom mocks using mockImplementations. Organize them using describe() and it(). Store mock files in a __mocks__ directory showing Jest where the Halloween party is.

6. Arrow functions for succinct mocks

Be a code minimalist:

jest.mock('myModule', () => ({
  myFunction: jest.fn().mockReturnValue("foobar"), // Because everyone loves unicorns
}));


7. Consistency matters

When declaring jest.mock(), don't sneak it into test cases. Place it at the top of the test file to ensure all imports are welcome by your jesters at the jest carnival, celebrating with jests and jesters.

Deep in the realm of mocking

1. Mastering dynamic behavior

mockImplementation() or mockImplementationOnce() are your secret weapons for dynamic mock behavior. It's like having a new trick up your sleeve for every magic show.

2. The order of tests

Test order is like a mixtape. The order can totally change the vibe. Use jest.resetAllMocks() to avoid your mixtape from turning into a remix.

3. Assertion expertise

Display your assertive nature with expect().toHaveBeenCalledTimes and expect().toHaveBeenCalledWith. So you can tell your module, "You are sure to talk exactly this many times with these specific words."

4. The mocks directory

Jest is an avid explorer. It automatically finds your mock files under a __mocks__ directory. Just name your mocks after the modules, and Jest will make the connection.