Jest is a popular JavaScript testing framework that is widely used in React projects. It provides a fast, simple, and powerful way to write and run tests for your applications. With Jest, you can ensure that your React components and logic work as expected, catch bugs early, and maintain confidence as your codebase grows. Jest is favored in the React ecosystem because it is easy to configure, comes with a rich set of features out of the box, and integrates smoothly with modern JavaScript tools. Its core features include a built-in test runner, assertion library, mocking capabilities, and support for code coverage reports.

The structure of a Jest test is straightforward but highly organized, making it easy for you to write clear and maintainable tests. At the top level, you often use a describe block to group related tests together. Inside a describe block, you write individual tests using either the test or it functions—these are interchangeable and define a single test case. Each test case contains one or more assertions, which are statements that check if your code behaves as expected. Assertions are made using the expect function, which takes a value and allows you to call matcher methods to compare the value to what you expect. For example, you might write expect(sum(1, 2)).toBe(3) to check that a sum function returns the correct result. This structure keeps your tests readable and focused, while making it easy to see what each test is verifying.

src/utils/sum.js

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

src/utils/sum.test.js

import { sum } from "./sum";

describe("sum utility function", () => {
  test("returns the correct sum of two numbers", () => {
    const result = sum(1, 2);

    expect(result).toBe(3);
  });
});