jest-testing-patterns

Jest Testing Patterns

Master Jest testing patterns including unit tests, mocks, spies, snapshots, and assertion techniques for comprehensive test coverage. This skill covers the fundamental patterns and practices for writing effective, maintainable tests using Jest.

Basic Test Structure

Test Suite Organization

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

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

    it('should handle zero', () => {
      expect(add(0, 5)).toBe(5);
    });
  });

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

Setup and Teardown

describe('Database operations', () => {
  let db;

  // Runs once before all tests in this describe block
  beforeAll(async () => {
    db = await initializeDatabase();
  });

  // Runs once after all tests in this describe block
  afterAll(async () => {
    await db.close();
  });

  // Runs before each test
  beforeEach(() => {
    db.clear();
  });

  // Runs after each test
  afterEach(() => {
    db.resetMocks();
  });

  it('should insert a record', async () => {
    const result = await db.insert({ name: 'John' });
    expect(result.id).toBeDefined();
  });

  it('should find a record', async () => {
    await db.insert({ id: 1, name: 'John' });
    const result = await db.findById(1);
    expect(result.name).toBe('John');
  });
});

Matchers and Assertions

Common Matchers

describe('Matchers', () => {
  it('should test equality', () => {
    expect(2 + 2).toBe(4); // Strict equality
    expect({ a: 1 }).toEqual({ a: 1 }); // Deep equality
    expect([1, 2, 3]).toStrictEqual([1, 2, 3]); // Strict deep equality
  });

  it('should test truthiness', () => {
    expect(true).toBeTruthy();
    expect(false).toBeFalsy();
    expect(null).toBeNull();
    expect(undefined).toBeUndefined();
    expect('value').toBeDefined();
  });

  it('should test numbers', () => {
    expect(4).toBeGreaterThan(3);
    expect(4).toBeGreaterThanOrEqual(4);
    expect(3).toBeLessThan(4);
    expect(3).toBeLessThanOrEqual(3);
    expect(0.1 + 0.2).toBeCloseTo(0.3, 5);
  });

  it('should test strings', () => {
    expect('team').not.toMatch(/I/);
    expect('Christoph').toMatch(/stop/);
    expect('hello world').toContain('world');
  });

  it('should test arrays and iterables', () => {
    const list = ['apple', 'banana', 'cherry'];
    expect(list).toContain('banana');
    expect(list).toHaveLength(3);
    expect(new Set(list)).toContain('apple');
  });

  it('should test objects', () => {
    expect({ a: 1, b: 2 }).toHaveProperty('a');
    expect({ a: 1, b: 2 }).toHaveProperty('a', 1);
    expect({ a: { b: { c: 1 } } }).toHaveProperty('a.b.c', 1);
  });

  it('should test exceptions', () => {
    expect(() => {
      throw new Error('error');
    }).toThrow();
    expect(() => {
      throw new Error('Invalid input');
    }).toThrow('Invalid input');
    expect(() => {
      throw new Error('Invalid input');
    }).toThrow(/Invalid/);
  });
});