Mocking ES6 Module Imports in Jest

Introduction

Jest provides a comprehensive suite of tools for unit testing, including mocking modules that are imported in your code. This article explains how to mock ES6 module imports in Jest, addressing the challenge of testing module dependencies.

Problem Statement

Mocking module imports allows developers to isolate the behavior of a specific module while executing tests for its dependent modules. However, the approach used in Jasmine, where imports are replaced with spies, is not directly applicable in Jest due to its different testing environment.

Solution

Jest provides the import * syntax to import all exports from a module as a single object. This technique can be leveraged to mock ES6 module imports.

Mocking Named Exports

For named exports, simply import the module using import * and then mutate the exported object to mock the desired function:

// dependency.js
export const doSomething = (y) => console.log(y);

// myModule.js
import { doSomething } from './dependency';

export default (x) => {
  doSomething(x * 2);
};

// myModule-test.js
import myModule from '../myModule';
import * as dependency from '../dependency';

describe('myModule', () => {
  it('calls the dependency with double the input', () => {
    dependency.doSomething = jest.fn(); // Mutate the named export

    myModule(2);

    expect(dependency.doSomething).toBeCalledWith(4);
  });
});

Mocking Default Exports

For default exports, you can import them using import moduleName from 'modulePath' and then mutate the default value of the imported object:

// dependency.js
export default (y) => console.log(y);

// myModule.js
import myModule from './myModule';
import * as dependency from '../dependency';

describe('myModule', () => {
  it('calls the dependency with double the input', () => {
    dependency.default = jest.fn(); // Mutate the default export

    myModule(2);

    expect(dependency.default).toBeCalledWith(4); // Assert against the default
  });
});

Conclusion

Using the import * syntax and mutating the exported object, it is possible to mock ES6 module imports in Jest, enabling you to test the functionality of your modules while isolating their dependencies.