Testing

Jest45.3k★

Jest is the testing framework that made JavaScript testing mainstream. Snapshot testing, mocking, coverage — all built in, zero config. At 45k stars and 20M+ weekly downloads, it's the most widely deployed JavaScript test runner. Every React tutorial assumes Jest. But Vitest is the future. 10-20x faster in watch mode, Jest-compatible API (most tests migrate without changes), native TypeScript support, and tight Vite integration. Bun's test runner is even faster but lacks jsdom and some Jest APIs. Both are better choices for new projects. Use Jest if your codebase already uses it and migration isn't worth the effort. The ecosystem of matchers, reporters, and plugins is massive. The catch: Jest is slow. Watch mode takes 3.4 seconds to re-run versus Vitest's 380ms. TypeScript requires additional configuration (ts-jest or SWC transform). The project's development has plateaued while Vitest iterates rapidly. For new projects in 2026, starting with Vitest is the clear call — you get Jest's API design with modern performance. Jest is for legacy, not new beginnings.

Selenium34.2k★

Selenium is the 20-year veteran of browser automation — the tool that defined the category and still runs test suites at most Fortune 500 companies. It supports every browser, every language, and every CI system. But in 2026, it's showing its age. Skip Selenium for new projects. Playwright is faster, more stable (92% vs Selenium's ~70% reliability rate), has better APIs, and handles modern web apps without the timing nightmares that plague Selenium tests. Cypress is solid for component testing but limited to Chromium and Firefox. If you're already deep in a Selenium suite, WebDriver BiDi in v4+ narrows the gap. The catch: Selenium's ubiquity is both its strength and trap. Millions of existing tests mean it's not going anywhere, but new features land slowly and the DX is a decade behind Playwright. If you're starting fresh, there's no reason to choose Selenium over Playwright in 2026. The ecosystem is mature; the tool is not.

k630.2k★

k6 is load testing that developers actually enjoy writing. JavaScript test scripts, not XML configs. Built-in protocols for HTTP, WebSocket, gRPC, and browser testing. Run locally, in CI, or scale to millions of requests with Grafana Cloud. It's the load testing tool that doesn't feel like punishment. If you need to verify your API can handle traffic before launch, k6 gets you there in an afternoon. JMeter is the Java dinosaur — powerful but miserable to use. Locust uses Python and is great for custom scenarios. Artillery is another JS option but less performant. Gatling uses Scala and is enterprise-heavy. Best for developers who want to add load testing to their CI pipeline without learning a new ecosystem. The JavaScript API means your frontend devs can write performance tests too. The catch: it's AGPL-3.0, which matters if you're embedding it in a product (not for running tests). The browser testing feature is newer and less mature. And serious distributed load testing requires Grafana Cloud k6 (paid) or setting up your own distributed runners. The free CLI is single-machine only.

Locust27.6k★

Locust lets you write load tests in plain Python — define user behavior as code, spin up thousands of simulated users, and watch your API crumble in real-time through a clean web UI. If you know Python, you can be load testing in 10 minutes. For Python developers who need to stress-test their APIs before launch, Locust is the fastest path. k6 (Grafana Labs) is the main competitor — JavaScript-based, slightly better performance, stronger in CI/CD pipelines. JMeter is the enterprise standard with GUI-driven test creation but XML configs and Java overhead. Gatling is Scala-based and fast. The catch: Locust is HTTP-focused — limited protocol support compared to JMeter (which handles SOAP, JMS, JDBC). The Python runtime means each worker simulates fewer concurrent users than k6's Go-based engine. And while the web UI is great for watching tests, the reporting/analytics are basic — you'll want to pipe results to Grafana for proper dashboards. Pick k6 if you want better CI integration.

React Testing Library19.6k★

React Testing Library changed how we think about component tests. Instead of testing implementation details — state, hooks, internal methods — you test what users see and do. Query by text, role, or label. Click buttons. Check visible output. If a refactor doesn't change the user experience, the tests still pass. That's the whole philosophy. Enzyme was the old way — shallow rendering and inspecting internal state. Testing Library won that war decisively. Playwright and Cypress handle end-to-end tests. Vitest or Jest run the test suite itself. Use React Testing Library for every React component test. It's not a test runner (pair it with Vitest or Jest), but it's the standard for writing tests that don't break when you refactor. MIT-licensed, 19K stars, used everywhere. The catch: if you genuinely need to test internal component state or hook behavior in isolation, Testing Library won't let you. That's by design — but some edge cases require reaching for lower-level tools. Also, the accessibility-first queries have a learning curve coming from Enzyme's selector-based approach.

Vitest16.2k★

Vitest is what happens when someone builds a test runner that actually understands your Vite project. Same config, same transforms, same speed. If you use Vite, switching to Vitest is a no-brainer — your tests run against the same pipeline as your app. The API is Jest-compatible, so migration is usually find-and-replace on imports. But Vitest is faster because it uses Vite's dev server for transforms instead of Babel. Watch mode is instant. TypeScript works without extra config. Component testing is built in. Jest is the incumbent — massive ecosystem, works everywhere, but slower and needs more config for modern setups. Testing Library pairs with both. Playwright and Cypress own E2E; Vitest owns unit and integration. Best for any project using Vite (which is most new projects). Solo founders get fast feedback loops without fighting configuration. The catch: if you're not on Vite, the advantage shrinks. Jest's ecosystem of custom matchers and reporters is still larger. And some edge cases around ESM/CJS module handling can bite you during migration.

Pytest13.7k★

Pytest won the Python testing wars, and it wasn't close. Where unittest makes you write Java-style boilerplate in Python, pytest lets tests look like regular functions with regular asserts. The plugin ecosystem (800+) covers everything from async testing to coverage to parallel execution. If you're writing Python — solo SaaS, API, data pipeline, anything — just use pytest. unittest ships with the standard library and costs nothing to try, but you'll fight its verbosity. Robot Framework targets BDD and non-technical teams, not developers. Nose2 exists but has a fraction of the community. Commercially, there's nothing worth paying for here. The real power is fixtures. Dependency injection for tests without the ceremony. Parametrize a single test across dozens of inputs in one decorator. The learning curve from "basic tests" to "advanced fixtures" is steeper than expected, but it pays off fast. The catch: pytest's magic (auto-discovery, fixture injection, conftest.py) can confuse newcomers. When a test fails because of fixture scoping, debugging feels like reading someone else's metaprogramming.

bombadil928★

Bombadil is property-based testing for web UIs — it randomly and systematically explores your app's state space, doing things you didn't think to test. Unexpected action sequences, weird timings, strange inputs. It finds the bugs your test suite misses because no human wrote a test for that exact combination. Built by Antithesis (the deterministic simulation company), the killer feature is perfect replay — when it finds a bug, you get the exact nanosecond-level event sequence that caused it. Write specs in TypeScript, run locally or in CI. Compared to Playwright (scripted paths), Bombadil finds bugs you didn't know to look for. Compared to fuzzing tools, it understands web UI semantics. Use this when your web app has complex state interactions and you've exhausted manual test scenarios. Skip this if your app is a simple CRUD — the setup overhead won't pay off. The catch: nascent (913 stars), requires thinking in properties rather than assertions, and the Antithesis platform integration is where the real power lives — the open-source local runner is the demo, not the product.