javascript-testing-expert_skill

Overview

This skill is an expert JavaScript testing assistant focused on writing high-quality unit and component tests that find bugs, document behavior, and prevent regressions. It emphasizes property-based testing with fast-check and defends tests against indeterministic code. It does not cover black-box end-to-end testing.

How this skill works

I inspect the code structure and existing test layout, then produce tests colocated and named to match your repository conventions (.spec.ts preferred). I recommend and adapt to tooling like vitest, fast-check, @fast-check/vitest, testing-library, msw and browser-playwright, and I generate example-based tests first followed by property-based tests for invariants and edge cases. I also flag test smells (excessive parameters, unstable values, network usage) and propose focused fixes or helpers.

When to use it

• When you need tests that act as documentation for function or component behavior
• When you want to find edge-case bugs that example tests miss using property-based testing
• When code under test is asynchronous or could suffer race conditions
• When tests are flaky due to indeterministic behavior (time, randomness, network)
• When a function is hard to test and you want a signal to refactor for single-responsibility

Best practices

• Mimic existing test structure and colocate one .spec.ts per source file, using describe per function and it starting with "should"
• Follow Arrange-Act-Assert (AAA) visibly in every it block and keep assertions focused and minimal
• Use real, realistic data for documentation-like tests and extract complex logic into testable small functions
• Favor stubs over mocks and never rely on network calls; use msw for HTTP stubbing
• Prefer property-based tests (fast-check) for "always/never" guarantees and use @fast-check/vitest integration when available
• Avoid heavy logic inside tests and warn when a function needs more than ~10 parameters or many mocks — recommend SRP refactor

Example use cases

• Write a suite for a date utility: simple examples plus property tests ensuring invariants across many dates and controlled system time
• Create tests for an async queue using fast-check scheduler to detect race conditions
• Add component tests that assert DOM structure and interactions with testing-library, plus optional screenshot/browser tests for visual regression
• Convert a brittle example test relying on randomness into a deterministic property-based test with faker wired into fast-check
• Replace network-dependent tests with msw-based stubs and add both example and property tests for edge-cases