Understanding Jest: A Beginner’s Guide
Introduction to Jest
What is Jest?
Jest is a delightful JavaScript Testing Framework with a focus on simplicity. It works out of the box for most JavaScript projects and is widely used in the React community.
Why Use Jest?
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Zero Configuration: Jest aims to work out of the box, config free, on most JavaScript projects.
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Great API: Provides a wide range of APIs to write tests in a more expressive and readable manner.
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Snapshot Testing: Supports snapshot testing to ensure UI does not change unexpectedly.
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Built-in Coverage Reports: Generates coverage reports for your code with minimal setup.
Setting Up Jest
Installing Jest
To start using Jest, you need to install it first. If you are using npm, run:
npm install --save-dev jestConfiguring Jest
In most cases, Jest works out of the box with minimal configuration. However, you can customize its behavior by adding a jest.config.js file in your project root.
Writing Your First Test
Create a file named sum.test.js. Inside it, write your first test:
const sum = require('./sum');
test('adds 1 + 2 to equal 3', () => {
expect(sum(1, 2)).toBe(3);
});
Advanced Topics
Mock Functions
Jest provides a way to create mock functions and spy on their behavior.
Asynchronous Testing
Jest can handle asynchronous code effortlessly, allowing for testing of promises, async/await functions, etc.
Snapshot Testing
Snapshot tests are a very useful tool whenever you want to make sure your UI does not change unexpectedly.
Coverage Reporting
Jest can collect code coverage information and report it in a variety of formats.
Conclusion
Jest is a powerful and easy-to-use testing framework that can help you write robust tests for your JavaScript applications.
Commonly Used Matchers in Jest with Examples
Jest provides a variety of “matchers” that let you validate different things in your tests. Here are some of the most commonly used matchers along with examples.
.toBe(value)
This matcher compares with ===. It’s used for exact equality.
Example:
test('two plus two is four', () => {
expect(2 + 2).toBe(4);
});
.toEqual(value)
This matcher is used for comparing the value of objects or arrays. It performs a deep equality check.
Example:
test('object assignment', () => {
const data = {one: 1};
data['two'] = 2;
expect(data).toEqual({one: 1, two: 2});
});
.toBeNull()
This matcher checks if a value is null.
Example:
test('null', () => {
const n = null;
expect(n).toBeNull();
expect(n).toBeDefined();
expect(n).not.toBeUndefined();
expect(n).not.toBeTruthy();
expect(n).toBeFalsy();
});
.toBeUndefined()
This matcher checks if a value is undefined.
Example:
test('undefined', () => {
const u = undefined;
expect(u).toBeUndefined();
});
.toBeDefined()
This matcher is the opposite of .toBeUndefined() and checks if a value is defined.
Example:
test('defined', () => {
const t = 'hello';
expect(t).toBeDefined();
});
.toBeTruthy()
This matcher checks if a value is truthy. Example:
test('true or false', () => {
expect(true).toBeTruthy();
});
.toBeFalsy()
This matcher checks if a value is falsy. Example:
test('false', () => {
expect(false).toBeFalsy();
});
.toBeGreaterThan(number)
This matcher checks if a value is greater than a certain number. Example:
test('greater than', () => {
const value = 10;
expect(value).toBeGreaterThan(9);
});
.toBeLessThan(number)
This matcher checks if a value is less than a certain number. Example:
test('less than', () => {
const value = 10;
expect(value).toBeLessThan(11);
});
.toBeCloseTo(number, numDigits)
This matcher is used for comparing floating point numbers. Example:
test('floating point numbers', () => {
const value = 0.2 + 0.1;
expect(value).toBeCloseTo(0.3, 5);
});
Conclusion
These are just a few of the matchers available in Jest. They provide a powerful way to write tests in a readable and expressive manner.