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How to Test JavaScript Code with Jest

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How to Test JavaScript Code with Jest

Testing code is crucial because it ensures the reliability, security, and proper functioning of products or systems, identifying potential issues before they become significant problems.

We just published a course on the freeCodeCamp.org YouTube channel that will teach you how to test JavaScirpt code with the Jest testing framework. Tomi Tokko created this course. He is an expereinced teacher and freeCodeCamp team member.

What is Jest?

Jest, developed by Facebook, is a delightful JavaScript Testing Framework with a focus on simplicity. It works seamlessly with projects using Babel, TypeScript, Node.js, React, Angular, and Vue.js, making it a versatile choice for a wide array of JavaScript projects.

Core Features of Jest

  • Zero Configuration: Jest is designed to work out of the box, with minimal setup required.
  • Snapshot Testing: This feature ensures your UI doesn’t change unexpectedly by capturing snapshots of your components.
  • Built-in Test Runner: Jest comes with a built-in test runner, eliminating the need for additional tools.
  • Mocking Support: It provides robust mocking capabilities, crucial for isolating tests from external dependencies.

Basic Jest Examples

Simple Test: Writing a basic test in Jest is straightforward. Here’s an example that tests a simple function:

function sum(a, b) {
  return a + b;
}

test('adds 1 + 2 to equal 3', () => {
  expect(sum(1, 2)).toBe(3);
});

This example defines a sum function and a test that checks if the function returns the expected result.

Asynchronous Test: Jest handles asynchronous code effortlessly. Here’s an example:

async function fetchData() {
  // Simulate fetching data from an API
  return 'freeCodeCamp';
}

test('the data is freeCodeCamp', async () => {
  const data = await fetchData();
  expect(data).toBe('freeCodeCamp');
});

This test ensures that the fetchData function resolves with the expected string.

Mock Functions: To test interactions within your code, you can use Jest’s mocking features:

const myMockFunction = jest.fn(x => x + 42);

test('mock function test', () => {
  expect(myMockFunction(0)).toBe(42);
});

This test checks that the mock function behaves as expected when called with a specific argument.

Course Content

Tomi’s course on freeCodeCamp’s YouTube channel is structured to cater to both beginners and experienced developers. It covers the basics, dives into advanced topics like mocking and snapshot testing, and provides real-world examples to solidify understanding.

Watch the full course on the freeCodeCamp.org YouTube channel (1-hour watch).

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