Master Mocha for Robust JavaScript Testing

This skill provides expert-level knowledge of Mocha, the feature-rich JavaScript test framework running on Node.js and in the browser, covering test structure, assertion libraries, mocking strategies, async testing patterns, and performance optimization. It covers proper test organization with nested describe/context/it blocks; setup/teardown hooks (before, beforeEach, afterEach, after); async testing via async/await or the done callback for non-promise operations; per-suite and per-test timeout management via this.timeout(); advanced Chai assertions including deep object matching, property-type checks, and custom assertion extensions (e.g. a validEmail matcher); Sinon-based stubbing with setup/restore in beforeEach/afterEach; a .mocharc.json configuration example (reporter, timeout, recursive spec globbing, parallel jobs); a shared test setup file wiring sinon-chai and global expect/sinon; exception testing for both async (rejectedWith) and synchronous (.to.throw) errors; and debugging aids like .only, .skip, and pending tests (an it() with no callback).

Use this skill when writing or reviewing Mocha test suites - organizing tests, wiring hooks, testing async code, extending Chai assertions, stubbing dependencies with Sinon, or configuring Mocha's runner and reporting. It is specific to Mocha's own APIs and conventions (not Jest or another test framework), and assumes Chai and Sinon as the assertion/mocking libraries shown throughout.

A .mocharc.json config sets reporter: "spec", timeout: 5000, recursive: true, a require array (e.g. test/setup.js), a spec glob (test/**/*.spec.js), an ignore glob for fixtures, exit: true, and parallel execution (parallel: true, jobs: 4). A shared test/setup.js sets NODE_ENV=test, wires chai.use(sinonChai), exposes global.expect/global.sinon, and resets Sinon stubs in a global beforeEach. package.json scripts commonly include mocha for a normal run, mocha --watch for watch mode, nyc mocha for coverage, and mocha --inspect-brk for debugging.

Integrates Chai for assertions (expect(...).to.deep.include(...), .to.have.property(...).that.is.a(...), custom chai.Assertion.addMethod extensions) and Sinon for stubbing/mocking (sinon.stub(paymentGateway, 'charge'), sinon.restore(), calledOnceWith assertions via sinon-chai). Documented performance/best-practice guidance: use beforeEach for test isolation rather than shared state, implement proper cleanup in afterEach, reserve before/after for expensive one-time setup, consider parallel execution for independent tests, and mock external dependencies consistently. Always structure tests with clear arrange-act-assert patterns, descriptive names that explain behavior, explicit error-scenario coverage, and consistent mocking strategy across the suite.

Developers writing or maintaining Mocha test suites who need concrete, working patterns for hooks, async tests, Chai/Sinon integration, custom assertions, timeout handling, and Mocha configuration rather than assembling them from scattered documentation. This is especially useful when a suite mixes promise-based async code with older callback-style operations (the done callback pattern) or needs custom domain-specific assertions (like the email-format matcher shown), since getting the hook ordering, timeout scope, and stub lifecycle wrong in those cases is a common source of flaky or order-dependent tests rather than a purely cosmetic style issue.