Mocha Testing Skill

Core Patterns

Basic Test with Chai

const { expect } = require('chai');

describe('Calculator', () => {
  let calc;
  beforeEach(() => { calc = new Calculator(); });

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

  it('should throw on divide by zero', () => {
    expect(() => calc.divide(10, 0)).to.throw('Division by zero');
  });
});

Chai Assertions

expect(value).to.equal(5);
expect(arr).to.have.lengthOf(3);
expect(obj).to.have.property('name');
expect(str).to.include('hello');
expect(fn).to.throw(Error);
expect(arr).to.deep.equal([1, 2, 3]);
expect(obj).to.deep.include({ name: 'Alice' });

Sinon Mocking

const sinon = require('sinon');

describe('UserService', () => {
  let sandbox;
  beforeEach(() => { sandbox = sinon.createSandbox(); });
  afterEach(() => { sandbox.restore(); });

  it('fetches user from API', async () => {
    const stub = sandbox.stub(api, 'get').resolves({ name: 'Alice' });
    const user = await userService.getUser(1);
    expect(user.name).to.equal('Alice');
    expect(stub.calledOnce).to.be.true;
  });
});

Async Testing

it('should fetch data', async () => {
  const data = await fetchData();
  expect(data).to.have.property('id');
});

it('callback style', (done) => {
  fetchData((err, data) => {
    expect(err).to.be.null;
    done();
  });
});

Anti-Patterns

| Bad | Good | Why | |-----|------|-----| | Missing done() | Use async/await | Hanging tests | | No sandbox | sinon.createSandbox() | Stubs leak | | Arrow in describe | Regular function for this.timeout() | Context |

Quick Reference

| Task | Command | |------|---------| | Run all | npx mocha | | Watch | npx mocha --watch | | Grep | npx mocha --grep "login" | | Timeout | npx mocha --timeout 10000 | | Recursive | npx mocha --recursive |

Setup: npm install mocha chai sinon --save-dev

Deep Patterns → reference/playbook.md

| § | Section | Lines | |---|---------|-------| | 1 | Production Configuration | mocharc, NYC coverage, TypeScript | | 2 | Testing with Chai + Sinon | Stubs, spies, assertions | | 3 | Advanced Sinon Patterns | Fake timers, nock HTTP, sequential | | 4 | Async Patterns | Promise, await, callback, events | | 5 | Hooks & Test Organization | Lifecycle, nesting, grep tags | | 6 | Custom Reporters & Plugins | Reporter class, root hooks | | 7 | Express/API Testing | Supertest integration | | 8 | CI/CD Integration | GitHub Actions, services | | 9 | Debugging Quick-Reference | 10 common problems | | 10 | Best Practices Checklist | 13 items |