Question 1

Mocha vs Jest — which should I choose?

Accepted Answer

Jest is all-in-one (bundler, runner, assertions, mocks) and dominates new projects. Mocha is modular — you choose Chai/Sinon/c8 separately — so it's lightweight and flexible. Pick Mocha if you need minimal dependencies (small CI footprint) or have legacy code. Pick Jest if starting fresh. Enterprise teams often stick with Mocha because refactoring 1000+ tests isn't worth the migration.

Question 2

How do I configure Chai assertions in Mocha?

Accepted Answer

Install chai: `npm install chai`. In test file: `const { expect } = require('chai');` then use `expect(value).to.equal(expected)`. Or use BDD style: `expect(x).to.be.true`, `expect(arr).to.include(item)`. Set assertion library in `.mocharc.json` via `require: ['chai/register-expect']` to avoid importing in every file.

Question 3

How do I test async/promise code in Mocha?

Accepted Answer

Return the promise: `it('should resolve', () => { return fetch(...).then(res => expect(res.ok).to.be.true) });`. Or use async/await: `it('async test', async () => { const res = await fetch(...); expect(res.ok).to.be.true; });`. Mocha waits for the returned promise. Set `timeout: 5000` if your async operation is slow. Never use arrow functions (loses this context); use `function() {}` or `() => {}` for lexically-bound promises.

Question 4

Can I run Mocha tests in parallel?

Accepted Answer

Yes, via `mocha-parallel-tests` npm package or Mocha's built-in `--parallel` flag (v8+). Parallel mode runs test files in separate processes. Warning: parallel tests need isolated state — if tests share a database or global state, they'll interfere. Use beforeEach/afterEach hooks to reset state per test. Expect 2-4x speedup on CI with 4+ cores.

Question 5

How do I set up ts-mocha for TypeScript tests?

Accepted Answer

Install: `npm install --save-dev ts-mocha typescript`. In `.mocharc.json`: `{ "require": "ts-node/register", "extensions": "ts", "spec": "test/**/*.spec.ts" }`. Or use ts-mocha directly: `ts-mocha --project tsconfig.json test/**/*.spec.ts`. Node will compile TS on the fly; slower than precompiled but great for dev.

Question 6

What goes in .mocharc.json and why?

Accepted Answer

`{ "require": "./setup.js", "recursive": true, "timeout": 5000, "reporter": "spec", "parallel": true }`. The require field loads a setup file before tests (env vars, DB seed). recursive = search folders. timeout = ms per test (increase for slow E2E). reporter = output format (spec/json/junit). parallel = run tests concurrently. Centralize here so CLI doesn't get crowded.

Question 7

When does Mocha beat Jest?

Accepted Answer

Mocha wins when: (1) you have legacy tests and Jest migration would be months of work, (2) you need a lightweight CI footprint (Mocha + minimal plugins vs Jest's monolithic install), (3) your team already knows Sinon mocking patterns, (4) you're testing browser code with a custom setup. Jest wins 90% of new projects. If you're starting fresh, use Jest or Vitest.

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