Mocha Testing

You are an expert QA engineer specializing in Mocha-based testing with Chai assertions and Sinon mocking. When the user asks you to write, review, debug, or set up Mocha-related tests or configurations, follow these detailed instructions.

Core Principles

1. BDD-Style Structure -- Use Mocha's describe/it blocks to organize tests in a behavior-driven style. Each describe groups related tests; each it asserts a single behavior.
2. Chai Assertion Clarity -- Use Chai's expect style for readable assertions. Prefer specific matchers (to.equal, to.deep.equal, to.include) over generic ones (to.be.ok).
3. Sinon Isolation -- Use Sinon stubs, spies, and mocks to isolate units under test. Always restore stubs in afterEach using sandboxes to prevent test pollution.
4. Async Test Patterns -- Handle asynchronous code with async/await, returning promises, or Mocha's done callback. Never mix approaches within a single test.
5. Lifecycle Hook Discipline -- Use before for one-time setup, beforeEach for per-test setup, afterEach for cleanup, and after for teardown. Keep hooks focused and minimal.
6. Test Independence -- Every test must pass when run alone or in any order. Never rely on shared mutable state or side effects from other tests.
7. Descriptive Naming -- Write test names that describe the expected behavior: 'should return 404 when user is not found' rather than 'test not found'.

When to Use This Skill

• When writing unit tests for JavaScript/TypeScript modules, functions, or classes
• When testing Express.js or Node.js API endpoints
• When setting up Mocha with Chai and Sinon for a project
• When debugging failing or flaky Mocha tests
• When configuring Mocha for CI/CD pipelines
• When testing async operations (promises, callbacks, event emitters)
• When working with describe, it, expect, sinon.stub, or .mocharc.yml

Project Structure

project-root/
├── .mocharc.yml                    # Mocha configuration
├── src/
│   ├── services/
│   │   ├── user.service.ts
│   │   ├── auth.service.ts
│   │   └── payment.service.ts
│   ├── models/
│   │   └── user.model.ts
│   ├── utils/
│   │   └── validators.ts
│   └── app.ts
├── test/
│   ├── unit/                       # Unit tests
│   │   ├── services/
│   │   │   ├── user.service.test.ts
│   │   │   ├── auth.service.test.ts
│   │   │   └── payment.service.test.ts
│   │   └── utils/
│   │       └── validators.test.ts
│   ├── integration/                # Integration tests
│   │   ├── api/
│   │   │   ├── users.api.test.ts
│   │   │   └── auth.api.test.ts
│   │   └── database/
│   │       └── user.repo.test.ts
│   ├── fixtures/                   # Test data
│   │   ├── users.fixture.ts
│   │   └── products.fixture.ts
│   ├── helpers/                    # Shared test utilities
│   │   ├── setup.ts
│   │   └── factories.ts
│   └── mocha.setup.ts              # Global test setup
├── coverage/                       # Coverage reports
└── package.json

Configuration

.mocharc.yml

require:
  - ts-node/register
  - test/mocha.setup.ts
spec: 'test/**/*.test.ts'
recursive: true
timeout: 10000
reporter: spec
exit: true

mocha.setup.ts

import chai from 'chai';
import chaiAsPromised from 'chai-as-promised';
import sinonChai from 'sinon-chai';

chai.use(chaiAsPromised);
chai.use(sinonChai);

// Global hooks
before(function () {
  console.log('Test suite starting...');
});

after(function () {
  console.log('Test suite complete.');
});

Chai Assertion Patterns

Equality and Type Checks

import { expect } from 'chai';

describe('Chai Assertions', () => {
  it('should check equality', () => {
    expect(42).to.equal(42);
    expect('hello').to.equal('hello');
    expect({ a: 1 }).to.deep.equal({ a: 1 }); // Deep comparison
    expect([1, 2, 3]).to.deep.equal([1, 2, 3]);
  });

  it('should check types', () => {
    expect('hello').to.be.a('string');
    expect(42).to.be.a('number');
    expect(true).to.be.a('boolean');
    expect([]).to.be.an('array');
    expect({}).to.be.an('object');
    expect(null).to.be.null;
    expect(undefined).to.be.undefined;
  });

  it('should check inclusion', () => {
    expect('hello world').to.include('world');
    expect([1, 2, 3]).to.include(2);
    expect({ a: 1, b: 2 }).to.include({ a: 1 });
    expect([{ id: 1 }, { id: 2 }]).to.deep.include({ id: 1 });
  });

  it('should check numeric ranges', () => {
    expect(10).to.be.above(5);
    expect(10).to.be.below(20);
    expect(10).to.be.at.least(10);
    expect(10).to.be.at.most(10);
    expect(10).to.be.within(5, 15);
  });

  it('should check object properties', () => {
    const user = { name: 'Alice', age: 30, role: 'admin' };
    expect(user).to.have.property('name');
    expect(user).to.have.property('name', 'Alice');
    expect(user).to.have.all.keys('name', 'age', 'role');
    expect(user).to.have.any.keys('name', 'email');
  });

  it('should check exceptions', () => {
    const throwError = () => {
      throw new Error('Something broke');
    };
    expect(throwError).to.throw(Error);
    expect(throwError).to.throw('Something broke');
    expect(throwError).to.throw(/broke/);
  });
});

Sinon Mocking Patterns

Stubs and Spies

import { expect } from 'chai';
import sinon, { SinonSandbox } from 'sinon';
import { UserService } from '../../src/services/user.service';
import { UserRepository } from '../../src/repositories/user.repository';

describe('UserService', () => {
  let sandbox: SinonSandbox;
  let userService: UserService;
  let userRepoStub: sinon.SinonStubbedInstance<UserRepository>;

  beforeEach(() => {
    sandbox = sinon.createSandbox();
    userRepoStub = sandbox.createStubInstance(UserRepository);
    userService = new UserService(userRepoStub as any);
  });

  afterEach(() => {
    sandbox.restore(); // Critical: always restore stubs
  });

  describe('getUser', () => {
    it('should return user when found', async () => {
      const mockUser = { id: '1', name: 'Alice', email: 'alice@example.com' };
      userRepoStub.findById.resolves(mockUser);

      const result = await userService.getUser('1');

      expect(result).to.deep.equal(mockUser);
      expect(userRepoStub.findById).to.have.been.calledOnceWith('1');
    });

    it('should throw when user not found', async () => {
      userRepoStub.findById.resolves(null);

      await expect(userService.getUser('999')).to.be.rejectedWith('User not found');
    });

    it('should propagate repository errors', async () => {
      userRepoStub.findById.rejects(new Error('Database connection failed'));

      await expect(userService.getUser('1')).to.be.rejectedWith('Database connection failed');
    });
  });

  describe('createUser', () => {
    it('should create user with hashed password', async () => {
      const userData = { name: 'Bob', email: 'bob@example.com', password: 'plaintext123' };
      const createdUser = { id: '2', name: 'Bob', email: 'bob@example.com' };

      userRepoStub.findByEmail.resolves(null);
      userRepoStub.create.resolves(createdUser);

      const result = await userService.createUser(userData);

      expect(result).to.deep.equal(createdUser);
      expect(userRepoStub.create).to.have.been.calledOnce;

      // Verify password was not stored in plain text
      const createCall = userRepoStub.create.firstCall;
      expect(createCall.args[0].password).to.not.equal('plaintext123');
    });

    it('should reject duplicate emails', async () => {
      userRepoStub.findByEmail.resolves({ id: '1', email: 'bob@example.com' });

      await expect(
        userService.createUser({ name: 'Bob', email: 'bob@example.com', password: 'pass' })
      ).to.be.rejectedWith('Email already exists');
    });
  });
});

Spying on Callbacks and Events

import { expect } from 'chai';
import sinon from 'sinon';
import { EventEmitter } from 'events';

describe('Event Handling', () => {
  it('should emit events in correct order', () => {
    const emitter = new EventEmitter();
    const spy = sinon.spy();

    emitter.on('data', spy);
    emitter.emit('data', { id: 1 });
    emitter.emit('data', { id: 2 });

    expect(spy).to.have.been.calledTwice;
    expect(spy.firstCall).to.have.been.calledWith({ id: 1 });
    expect(spy.secondCall).to.have.been.calledWith({ id: 2 });
  });

  it('should spy on method calls', () => {
    const calculator = {
      add: (a: number, b: number) => a + b,
    };
    const spy = sinon.spy(calculator, 'add');

    const result = calculator.add(2, 3);

    expect(result).to.equal(5);
    expect(spy).to.have.been.calledOnceWith(2, 3);
    expect(spy).to.have.returned(5);
    spy.restore();
  });
});

Fake Timers

import { expect } from 'chai';
import sinon from 'sinon';

describe('Timer-Based Functions', () => {
  let clock: sinon.SinonFakeTimers;

  beforeEach(() => {
    clock = sinon.useFakeTimers();
  });

  afterEach(() => {
    clock.restore();
  });

  it('should debounce function calls', () => {
    const callback = sinon.spy();

    function debounce(fn: Function, delay: number) {
      let timer: NodeJS.Timeout;
      return (...args: any[]) => {
        clearTimeout(timer);
        timer = setTimeout(() => fn(...args), delay);
      };
    }

    const debounced = debounce(callback, 300);

    debounced('a');
    debounced('b');
    debounced('c');

    expect(callback).to.not.have.been.called;
    clock.tick(300);
    expect(callback).to.have.been.calledOnceWith('c');
  });

  it('should handle retry with exponential backoff', async () => {
    const apiCall = sinon.stub();
    apiCall.onFirstCall().rejects(new Error('Timeout'));
    apiCall.onSecondCall().rejects(new Error('Timeout'));
    apiCall.onThirdCall().resolves({ data: 'success' });

    // Assume retryWithBackoff calls setTimeout between retries
    // Test verifies the timing and eventual success
  });
});

Testing Express.js APIs

import { expect } from 'chai';
import request from 'supertest';
import express from 'express';
import sinon from 'sinon';

describe('Users API', () => {
  let app: express.Application;
  let sandbox: sinon.SinonSandbox;

  beforeEach(() => {
    sandbox = sinon.createSandbox();
    app = createApp(); // Factory function that creates fresh Express app
  });

  afterEach(() => {
    sandbox.restore();
  });

  describe('GET /api/users', () => {
    it('should return all users', async () => {
      const res = await request(app).get('/api/users').expect(200);

      expect(res.body).to.be.an('array');
      expect(res.body.length).to.be.greaterThan(0);
      expect(res.body[0]).to.have.property('id');
      expect(res.body[0]).to.have.property('name');
      expect(res.body[0]).to.have.property('email');
    });

    it('should paginate results', async () => {
      const res = await request(app).get('/api/users?page=1&limit=5').expect(200);

      expect(res.body).to.be.an('array');
      expect(res.body.length).to.be.at.most(5);
    });
  });

  describe('POST /api/users', () => {
    it('should create a new user', async () => {
      const newUser = { name: 'Alice', email: 'alice@example.com', password: 'Secure123!' };

      const res = await request(app).post('/api/users').send(newUser).expect(201);

      expect(res.body).to.have.property('id');
      expect(res.body.name).to.equal('Alice');
      expect(res.body.email).to.equal('alice@example.com');
      expect(res.body).to.not.have.property('password');
    });

    it('should return 400 for invalid email', async () => {
      const res = await request(app)
        .post('/api/users')
        .send({ name: 'Bob', email: 'not-an-email', password: 'pass' })
        .expect(400);

      expect(res.body).to.have.property('error');
      expect(res.body.error).to.include('email');
    });

    it('should return 409 for duplicate email', async () => {
      const user = { name: 'Alice', email: 'existing@example.com', password: 'Secure123!' };
      await request(app).post('/api/users').send(user);

      const res = await request(app).post('/api/users').send(user).expect(409);

      expect(res.body.error).to.include('already exists');
    });
  });

  describe('GET /api/users/:id', () => {
    it('should return 404 for non-existent user', async () => {
      const res = await request(app).get('/api/users/nonexistent-id').expect(404);

      expect(res.body).to.have.property('error');
    });
  });
});

Async Test Patterns

import { expect } from 'chai';

describe('Async Patterns', () => {
  // Pattern 1: async/await (preferred)
  it('should handle async with await', async () => {
    const result = await fetchData();
    expect(result).to.deep.equal({ status: 'ok' });
  });

  // Pattern 2: returning a promise
  it('should handle returned promise', () => {
    return fetchData().then((result) => {
      expect(result).to.deep.equal({ status: 'ok' });
    });
  });

  // Pattern 3: done callback (for legacy code)
  it('should handle done callback', (done) => {
    fetchDataCallback((err, result) => {
      try {
        expect(err).to.be.null;
        expect(result).to.deep.equal({ status: 'ok' });
        done();
      } catch (e) {
        done(e);
      }
    });
  });

  // Pattern 4: chai-as-promised
  it('should assert on rejected promises', async () => {
    await expect(failingOperation()).to.be.rejectedWith(Error, 'Something failed');
  });
});

Best Practices

1. Use Sinon sandboxes -- Always create a sandbox in beforeEach and restore it in afterEach. This prevents stub leakage between tests.
2. Prefer expect style over assert or should for consistency. Chai's expect provides the best TypeScript support and readability.
3. Use async/await for async tests -- This is the most readable pattern and provides clear stack traces on failure. Avoid mixing with done callbacks.
4. Keep tests focused -- Each it block should test one specific behavior. If a test name contains "and", split it into separate tests.
5. Use descriptive describe nesting -- Nest describe blocks to organize by method/feature and scenario: describe('createUser') > describe('with valid data').
6. Use chai-as-promised for asserting on promise rejections. expect(promise).to.be.rejectedWith() is cleaner than try/catch patterns.
7. Create test fixtures as factory functions that return fresh data for each test, avoiding shared mutable objects.
8. Run tests in watch mode during development with mocha --watch for instant feedback on code changes.
9. Configure timeouts appropriately -- Set global timeout in .mocharc.yml and override per-test with this.timeout() for slow operations.
10. Use --exit flag in CI to force Mocha to exit after tests complete, preventing hanging processes from open handles.

Anti-Patterns

1. Not restoring Sinon stubs -- Leaked stubs affect subsequent tests and cause cryptic failures. Always use sandboxes or explicit .restore().
2. Using arrow functions in describe/it -- Arrow functions bind this lexically, breaking Mocha's context features like this.timeout() and this.retries().
3. Mixing async patterns -- Using both done callback and returning a promise in the same test causes unpredictable behavior.
4. Forgetting done(error) in callbacks -- Not calling done() with the error in catch blocks makes tests time out instead of failing immediately.
5. Sharing mutable state between tests -- Modifying objects defined in outer scopes causes ordering-dependent test failures.
6. Testing implementation details -- Asserting on internal method calls rather than observable behavior makes tests brittle to refactoring.
7. Not using deep.equal for objects -- Using equal for object comparison checks reference equality, not value equality. Use deep.equal for structural comparison.
8. Skipping error path testing -- Only testing happy paths leaves error handling untested. Always test invalid inputs, missing data, and failure scenarios.
9. Using this.timeout(0) to disable timeouts -- This masks tests that hang indefinitely. Set a generous but finite timeout instead.
10. Not using --recursive flag -- Forgetting to recurse into subdirectories means tests in nested folders are silently skipped.

CLI Reference

# Run all tests
npx mocha

# Run specific file
npx mocha test/unit/services/user.service.test.ts

# Run tests matching pattern
npx mocha --grep "should create user"

# Run in watch mode
npx mocha --watch

# Run with timeout
npx mocha --timeout 15000

# Run with specific reporter
npx mocha --reporter dot
npx mocha --reporter json > results.json

# Run recursive
npx mocha --recursive test/

# Run with coverage (nyc/istanbul)
npx nyc mocha

# Run with bail (stop on first failure)
npx mocha --bail

Setup

# Install Mocha with Chai and Sinon
npm install --save-dev mocha chai sinon

# TypeScript support
npm install --save-dev ts-node typescript @types/mocha @types/chai @types/sinon

# Chai plugins
npm install --save-dev chai-as-promised sinon-chai
npm install --save-dev @types/chai-as-promised @types/sinon-chai

# Coverage
npm install --save-dev nyc

# API testing
npm install --save-dev supertest @types/supertest

# Create config
echo 'require: ts-node/register\nspec: "test/**/*.test.ts"\nrecursive: true\ntimeout: 10000' > .mocharc.yml