Testing

A Chai.js assertion reference covering all three styles — expect and should (BDD) and assert (TDD): equality, deep comparison, types, inclusion, thrown errors, numeric bounds, plus async and HTTP helpers via plugins. Copy and run.

A Sinon.js test-double reference: spies that observe calls, stubs that replace behavior with returns/throws/resolves, strict mocks, fake timers, sandboxes, and assertion helpers — the tools for isolating units under test. Copy and run.

Jest testing APIs covering test cases and grouping (describe/it), matchers and assertions, async testing, mock functions and module mocking, snapshot tests, and coverage reports. For writing JavaScript unit tests that fail for the right reason.