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