jest

Jest Testing Framework

Instructions

1. Assess the testing need: Determine if it's unit tests, integration tests, or snapshot tests.
2. Follow Jest conventions:
   • Test files: *.test.ts, *.spec.ts, or in __tests__/ directory
   • Use describe blocks to group related tests
   • Use it or test for individual test cases
   • Use beforeEach/afterEach for setup/teardown
3. Provide complete examples: Include imports, mocks, and assertions.
4. Guide on mocking: Explain jest.mock(), jest.fn(), and jest.spyOn().
5. Cover edge cases: Test error conditions, boundary values, and async behavior.

Test Structure

import { describe, it, expect, beforeEach, jest } from '@jest/globals';
import { myFunction } from './myModule';

describe('myFunction', () => {
  beforeEach(() => {
    jest.clearAllMocks();
  });

  it('should return expected value for valid input', () => {
    const result = myFunction('valid');
    expect(result).toBe('expected');
  });

  it('should throw error for invalid input', () => {
    expect(() => myFunction(null)).toThrow('Invalid input');
  });

  it('should handle async operations', async () => {
    const result = await myFunction('async');
    expect(result).resolves.toBe('done');
  });
});

Mocking Patterns

Mock a module

jest.mock('./database', () => ({
  query: jest.fn().mockResolvedValue([{ id: 1 }]),
}));

Mock a function

const mockCallback = jest.fn((x) => x + 1);
expect(mockCallback(1)).toBe(2);
expect(mockCallback).toHaveBeenCalledWith(1);

Spy on methods

const spy = jest.spyOn(object, 'method');
object.method();
expect(spy).toHaveBeenCalled();
spy.mockRestore();

Mock timers

jest.useFakeTimers();
setTimeout(callback, 1000);
jest.advanceTimersByTime(1000);
expect(callback).toHaveBeenCalled();

Common Assertions

// Equality
expect(value).toBe(exact);           // Strict equality
expect(value).toEqual(object);       // Deep equality
expect(value).toStrictEqual(object); // Deep + type equality

// Truthiness
expect(value).toBeTruthy();
expect(value).toBeFalsy();
expect(value).toBeNull();
expect(value).toBeUndefined();
expect(value).toBeDefined();

// Numbers
expect(value).toBeGreaterThan(3);
expect(value).toBeLessThanOrEqual(5);
expect(value).toBeCloseTo(0.3, 5);   // Floating point

// Strings
expect(string).toMatch(/pattern/);
expect(string).toContain('substring');

// Arrays
expect(array).toContain(item);
expect(array).toHaveLength(3);

// Objects
expect(object).toHaveProperty('key', 'value');
expect(object).toMatchObject({ partial: 'match' });

// Errors
expect(() => fn()).toThrow();
expect(() => fn()).toThrow('message');
expect(() => fn()).toThrow(ErrorClass);

// Async
await expect(promise).resolves.toBe(value);
await expect(promise).rejects.toThrow();