# Jest Quick Reference

*Tests, matchers, mocking, async, and snapshots*

> Source: Jest Documentation (jestjs.io) · MIT

## 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

| Command | Description |
|---------|-------------|
| `*.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

| Command | Description |
|---------|-------------|
| `test.skip()` | Skip this test |
| `test.only()` | Run only this test |
| `describe.skip()` | Skip entire suite |
| `describe.only()` | Run only this suite |

## Matchers

### Equality

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

### Truthiness

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

### Numbers

| Command | Description |
|---------|-------------|
| `.toBeGreaterThan(n)` | Greater than n |
| `.toBeLessThanOrEqual(n)` | Less than or equal |
| `.toBeCloseTo(0.3, 5)` | Float comparison (5 digits) |

### Strings, Arrays, Objects

| Command | Description |
|---------|-------------|
| `.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

| Command | Description |
|---------|-------------|
| `.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(<App />).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

| Command | Description |
|---------|-------------|
| `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}]` };
}});
```