How to Handle ES6 Module Imports Mocking with Jest?

Mocking ES6 Module Imports with Jest

In the world of testing with ES6 modules, mocking dependencies can sometimes pose a challenge. This article delves into how to mock ES6 module imports using Jest, providing a solution that works for both named and default exports.

The Problem

Consider the following hypothetical ES6 module that relies on another module for functionality:

<code class="js">// myModule.js
import dependency from './dependency';

export default (x) => {
  dependency.doSomething(x * 2);
};</code>

In an ideal testing scenario, we'd like to mock the dependency module and assert that the doSomething function is being called with the expected arguments. However, this task can be tricky with Jest.

The 'Yuck' Approach

A common approach involves replacing imports with requires and moving them within tests, as seen here:

<code class="js">// Dependency Code
export const doSomething = (y) => console.log(y);

// Module Code (Modified)
export default (x) => {
  const dependency = require('./dependency'); // Yuck
  dependency.doSomething(x * 2);
};

// Test Code (Modified)
import myModule from '../myModule';
import dependency from '../dependency';

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

    myModule(2);

    const dependency = require('../dependency'); // Also yuck
    expect(dependency.doSomething).toBeCalledWith(4);
  });
});</code>

While this approach satisfies the immediate need, it introduces unnecessary modifications to the codebase and degrades the overall code quality.

The 'Hack' Approach

A more elegant solution involves using the import * syntax. This technique allows direct mutation of named or default exports, making mocking effortless.

For Named Exports:

<code class="js">// Test Code
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);
  });
});</code>

For Default Exports:

<code class="js">// Test Code
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
  });
});
</code>

Caution

It is important to note that mutating imported modules in such a manner can lead to side effects and unpredictable behavior in tests. Therefore, while this approach may provide a quick fix, it is not recommended as a long-term practice. Jest provides more conventional methods for mocking and spying on modules, which should be considered instead.

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