Jest

JavaScript testing framework with built-in mocking.

When to Use

• Unit testing JavaScript/TypeScript
• React component testing
• Snapshot testing
• Mocking dependencies

Quick Start

// sum.test.ts
import { sum } from "./sum";

describe("sum", () => {
  it("adds two numbers", () => {
    expect(sum(1, 2)).toBe(3);
  });

  it("handles negative numbers", () => {
    expect(sum(-1, 1)).toBe(0);
  });
});

Core Concepts

Matchers

// Common matchers
expect(value).toBe(expected); // Strict equality
expect(value).toEqual(expected); // Deep equality
expect(value).toBeTruthy();
expect(value).toBeFalsy();
expect(value).toBeNull();
expect(value).toBeUndefined();
expect(array).toContain(item);
expect(string).toMatch(/regex/);
expect(fn).toThrow(Error);
expect(promise).resolves.toBe(value);
expect(promise).rejects.toThrow();

Mocking

import { fetchUser } from "./api";
import { UserService } from "./UserService";

jest.mock("./api");

const mockedFetchUser = fetchUser as jest.MockedFunction<typeof fetchUser>;

describe("UserService", () => {
  beforeEach(() => {
    jest.clearAllMocks();
  });

  it("fetches user data", async () => {
    mockedFetchUser.mockResolvedValue({ id: "1", name: "John" });

    const service = new UserService();
    const user = await service.getUser("1");

    expect(user.name).toBe("John");
    expect(mockedFetchUser).toHaveBeenCalledWith("1");
  });
});

Common Patterns

Async Testing

it("handles async operations", async () => {
  const result = await fetchData();
  expect(result).toBeDefined();
});

it("handles promises", () => {
  return expect(asyncFn()).resolves.toBe("value");
});

it("handles callbacks", (done) => {
  callback((result) => {
    expect(result).toBe("value");
    done();
  });
});

Snapshot Testing

import { render } from '@testing-library/react';
import { Button } from './Button';

it('matches snapshot', () => {
  const { container } = render(<Button label="Click me" />);
  expect(container).toMatchSnapshot();
});

Best Practices

Do:

• Use descriptive test names
• Mock external dependencies
• Test edge cases
• Keep tests isolated

Don't:

• Test implementation details
• Share state between tests
• Use too many snapshots
• Skip cleanup

Troubleshooting

Issue	Cause	Solution
Test timeout	Async not awaited	Add await or done()
Mock not working	Wrong path	Check module path
Flaky tests	Shared state	Use beforeEach cleanup

References

• Jest Documentation
• Testing Library