Setup

Installation

npm install --save-dev jest
# package.json: "scripts": { "test": "jest" }
npx jest              # run all tests
npx jest --watch      # re-run on changes

File Naming

*.test.js	Test files (default pattern)
*.spec.js	Alternative test pattern
__tests__/	Test directory (auto-discovered)

Running Specific Tests

npx jest path/to/file.test.js
npx jest --testNamePattern="adds"
npx jest --verbose     # detailed output

Basic Tests

Test Structure

describe("Calculator", () => {
  test("adds 1 + 2 to equal 3", () => {
    expect(add(1, 2)).toBe(3);
  });
});

test vs it

test("works correctly", () => { /* ... */ });
it("should work correctly", () => { /* ... */ });
// Both are identical; "it" reads like English

Skipping & Focusing

test.skip()	Skip this test
test.only()	Run only this test
describe.skip()	Skip entire suite
describe.only()	Run only this suite

Matchers

Equality

.toBe(val)	Strict equality (`===`)
.toEqual(val)	Deep equality (objects/arrays)
.toStrictEqual(val)	Deep + type + undefined props
.not.toBe(val)	Negate any matcher

Truthiness

.toBeTruthy()	Truthy value
.toBeFalsy()	Falsy value
.toBeNull()	Exactly `null`
.toBeUndefined()	Exactly `undefined`
.toBeDefined()	Not `undefined`

Numbers

.toBeGreaterThan(n)	Greater than n
.toBeLessThanOrEqual(n)	Less than or equal
.toBeCloseTo(0.3, 5)	Float comparison (5 digits)

Strings, Arrays, Objects

.toMatch(/regex/)	String matches regex
.toContain(item)	Array/iterable contains item
.toHaveLength(n)	Length of array/string
.toHaveProperty(key, val)	Object has property
.toMatchObject(obj)	Object contains subset

Async Testing

async / await

test("fetches data", async () => {
  const data = await fetchData();
  expect(data).toEqual({ id: 1 });
});

Promises

test("resolves to data", () => {
  return expect(fetchData())
    .resolves.toEqual({ id: 1 });
});

Rejections

test("rejects with error", async () => {
  await expect(fetchBad())
    .rejects.toThrow("Not Found");
});

Exceptions

test("throws on invalid input", () => {
  expect(() => validate(null)).toThrow();
  expect(() => validate(null)).toThrow("invalid");
});

Mocking

Mock Functions

const fn = jest.fn();
fn("hello");
expect(fn).toHaveBeenCalledWith("hello");
expect(fn).toHaveBeenCalledTimes(1);

Mock Return Values

const fn = jest.fn()
  .mockReturnValue(42)
  .mockReturnValueOnce(99);
fn(); // 99 (first call)
fn(); // 42 (subsequent)

Mocking Modules

jest.mock("./api");
const { fetchUser } = require("./api");
fetchUser.mockResolvedValue({ name: "Alice" });

Mock Matchers

.toHaveBeenCalled()	Called at least once
.toHaveBeenCalledTimes(n)	Called exactly n times
.toHaveBeenCalledWith(args)	Called with specific arguments
.toHaveBeenLastCalledWith(args)	Last call had these arguments

Spies

Spying on Methods

const spy = jest.spyOn(Math, "random")
  .mockReturnValue(0.5);
expect(Math.random()).toBe(0.5);
spy.mockRestore();  // restore original

Spying on Object Methods

const obj = { greet: (n) => `Hi ${n}` };
const spy = jest.spyOn(obj, "greet");
obj.greet("Alice");
expect(spy).toHaveBeenCalledWith("Alice");

Snapshots

Snapshot Testing

test("renders correctly", () => {
  const tree = renderer.create().toJSON();
  expect(tree).toMatchSnapshot();
});

Inline Snapshots

test("formats name", () => {
  expect(formatName("alice"))
    .toMatchInlineSnapshot(`"Alice"`);
});

Updating Snapshots

npx jest --updateSnapshot   # update all
npx jest --updateSnapshot --testNamePattern="renders"

Setup & Teardown

Lifecycle Hooks

beforeAll(() => { /* once before all tests */ });
afterAll(() => { /* once after all tests */ });
beforeEach(() => { /* before each test */ });
afterEach(() => { /* after each test */ });

Scoping

describe("Database", () => {
  beforeEach(() => db.connect());
  afterEach(() => db.disconnect());
  test("reads data", () => { /* ... */ });
});

Hooks inside describe only apply to that block

Configuration

jest.config.js

module.exports = {
  testEnvironment: "node",
  coverageThreshold: {
    global: { branches: 80, lines: 80 }
  },
};

Common Options

testEnvironment	`"node"` or `"jsdom"` (DOM)
roots	Directories to search for tests
collectCoverage	Enable coverage reporting
coverageDirectory	Output directory for coverage
moduleNameMapper	Path aliases (e.g., `@/` prefix)
transform	File transforms (Babel, TS, etc.)
setupFilesAfterFramework	Run setup before each suite

Coverage

npx jest --coverage
npx jest --collectCoverageFrom="src/**/*.js"

Common Patterns

Testing API Calls

jest.mock("./api");
test("loads users", async () => {
  api.getUsers.mockResolvedValue([{id: 1}]);
  const users = await loadUsers();
  expect(users).toHaveLength(1);
});

Timer Mocks

jest.useFakeTimers();
test("delays execution", () => {
  const cb = jest.fn();
  setTimeout(cb, 1000);
  jest.advanceTimersByTime(1000);
  expect(cb).toHaveBeenCalled();
});

Parameterized Tests

test.each([
  [1, 1, 2],
  [2, 3, 5],
])("add(%i, %i) = %i", (a, b, expected) => {
  expect(add(a, b)).toBe(expected);
});

Custom Matchers

expect.extend({
  toBeWithinRange(received, floor, ceil) {
    const pass = received >= floor
      && received <= ceil;
    return { pass, message: () =>
      `expected ${received} in [${floor},${ceil}]` };
}});

JEST QUICK REFERENCE