Jest can be installed via npm or yarn:

npm install --save-dev jest
# or
yarn add --dev jest

Basic setup in package.json:

{
  "scripts": {
    "test": "jest",
    "test:watch": "jest --watch",
    "test:coverage": "jest --coverage"
  }
}

For projects without Babel, you might need additional configuration for ES6+ syntax.

There is no functional difference between test() and it() - they are aliases of each other. Both are used to define individual test cases:

test('should add two numbers', () => {
  expect(add(2, 3)).toBe(5);
});

it('should add two numbers', () => {
  expect(add(2, 3)).toBe(5);
});

The choice between them is often stylistic. it() reads more naturally in BDD (Behavior Driven Development) style, while test() is more explicit about its purpose.

describe() is used to group related tests together. It creates a test suite and helps organize tests logically:

describe('Calculator', () => {
  test('should add two numbers', () => {
    expect(add(2, 3)).toBe(5);
  });
  
  test('should subtract two numbers', () => {
    expect(subtract(5, 3)).toBe(2);
  });
});

Benefits of using describe():

• Better test organization and readability
• Scoped setup and teardown hooks
• Grouped test output in reports
• Easier to run specific test suites

A well-structured Jest test file typically follows this pattern:

// Import dependencies
import { functionToTest } from '../src/utils';

// Describe the module/component being tested
describe('functionToTest', () => {
  // Group related tests
  describe('when given valid input', () => {
    test('should return expected result', () => {
      // Arrange
      const input = 'test';
      
      // Act
      const result = functionToTest(input);
      
      // Assert
      expect(result).toBe('expected');
    });
  });
  
  describe('when given invalid input', () => {
    test('should throw an error', () => {
      expect(() => functionToTest(null)).toThrow();
    });
  });
});

Jest can be run in several ways:

1. Basic test run: npm test or jest
2. Watch mode: jest --watch (watches for file changes)
3. Watch all: jest --watchAll (watches all files)
4. Coverage: jest --coverage (generates coverage report)
5. Specific file: jest mytest.test.js
6. Pattern matching: jest --testNamePattern="should add"
7. Verbose output: jest --verbose

Common package.json scripts:

{
  "scripts": {
    "test": "jest",
    "test:watch": "jest --watch",
    "test:coverage": "jest --coverage",
    "test:debug": "jest --verbose"
  }
}

Jest provides many built-in matchers for different assertion types:

Equality matchers:

• toBe(): Exact equality (Object.is)
• toEqual(): Deep equality
• toStrictEqual(): Strict deep equality

Truthiness matchers:

• toBeTruthy(): Truthy values
• toBeFalsy(): Falsy values
• toBeNull(): Specifically null
• toBeUndefined(): Specifically undefined

Number matchers:

• toBeGreaterThan(): Greater than
• toBeCloseTo(): Floating point numbers

String matchers:

• toMatch(): Regular expressions
• toContain(): Substring matching

Array/Object matchers:

• toContain(): Array contains item
• toHaveProperty(): Object has property

toBe() uses Object.is() for exact equality (reference equality for objects), while toEqual() performs deep equality checking:

test('toBe vs toEqual', () => {
  const obj1 = { name: 'John' };
  const obj2 = { name: 'John' };
  const obj3 = obj1;
  
  // toBe checks reference equality
  expect(obj1).toBe(obj3); // ✓ Same reference
  expect(obj1).toBe(obj2); // ✗ Different references
  
  // toEqual checks deep equality
  expect(obj1).toEqual(obj2); // ✓ Same content
  expect(obj1).toEqual(obj3); // ✓ Same content
  
  // For primitives, they work similarly
  expect(5).toBe(5); // ✓
  expect(5).toEqual(5); // ✓
});