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How to Fix “fetch is not defined” Error in Jest Tests for JavaScript Applications

Anastasios Antoniadis

Share on X (Twitter) Share on Facebook Share on Pinterest Share on LinkedInTesting is a critical phase in the software development lifecycle, ensuring that your code behaves as expected. When using Jest, a popular testing framework for JavaScript, to test code that involves fetching data from APIs, developers might encounter an error stating “fetch is …

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Testing is a critical phase in the software development lifecycle, ensuring that your code behaves as expected. When using Jest, a popular testing framework for JavaScript, to test code that involves fetching data from APIs, developers might encounter an error stating “fetch is not defined.” This error occurs because Jest, which runs in a Node.js environment, does not have a built-in fetch function, unlike browsers. This article will guide you through understanding this issue and provide detailed solutions to fix it, enabling efficient testing of your JavaScript applications that use the fetch API.

Understanding the Issue

The fetch API provides a modern, promise-based mechanism to make HTTP requests in the browser. However, when running tests with Jest in a Node.js environment, the fetch function is not available by default, leading to the “fetch is not defined” error. This discrepancy arises because Node.js and browsers have different built-in globals and APIs.

Common Causes

  • Testing Client-Side Code: Attempting to test code designed to run in the browser, which relies on the fetch API, without mocking or polyfilling fetch in the Jest environment.
  • Environment Configuration: Not configuring the Jest environment to emulate browser-like features that include the fetch API.

How to Fix the Error

To resolve the “fetch is not defined” error in Jest tests, you can either mock the fetch function or use a polyfill to add fetch support to the Node.js environment used by Jest. Below are methods to achieve this:

1. Mocking fetch Using Jest

Mocking the fetch API allows you to simulate the behavior of fetch requests without making actual network calls. This approach is useful for unit testing and when you want to control the responses of fetch calls.

Example of Mocking fetch:

// In your Jest test file
global.fetch = jest.fn(() =>
  Promise.resolve({
    json: () => Promise.resolve({ data: 'mocked data' }),
  })
);

// Remember to clear the mock in between tests
beforeEach(() => {
  fetch.mockClear();
});

2. Using a fetch Polyfill

Another solution is to use a polyfill that implements the fetch function in Node.js. node-fetch is a popular package that brings fetch to Node.js, making it available in Jest tests.

Step 1: Install node-fetch

npm install node-fetch

Step 2: Polyfill fetch in Jest

You can polyfill fetch globally for all tests or on a per-test basis.

  • Globally in Jest Setup File:

Create or edit your Jest setup file (specified in jest.config.js or package.json as setupFilesAfterEnv) and add the following:

// jest.setup.js
global.fetch = require('node-fetch');
  • Per-Test Basis:

Alternatively, you can require and assign node-fetch at the beginning of test files or individual tests that require fetch.

const fetch = require('node-fetch');
global.fetch = fetch;

3. Using Jest Environment Options

For a more integrated solution, consider using a Jest environment that emulates a browser-like environment, such as jest-environment-jsdom. This approach can provide broader API support beyond just fetch.

Configuration in jest.config.js:

module.exports = {
  testEnvironment: 'jest-environment-jsdom',
};

Ensure you have jsdom and any necessary polyfills configured to support fetch.

Conclusion

The “fetch is not defined” error in Jest reflects the environment discrepancy between browsers and Node.js, particularly concerning the availability of the fetch API. By choosing to mock fetch calls or using a polyfill like node-fetch, developers can effectively bypass this issue, enabling the testing of asynchronous operations involving HTTP requests in Jest. Each solution has its context where it shines, from mocking for isolated unit tests to polyfilling for more integrated or end-to-end testing scenarios. Properly handling fetch in Jest tests ensures your JavaScript applications behave as expected, both in development and production environments.

Anastasios Antoniadis
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