Unit testing guides — write fast, reliable unit tests with Jest, Vitest, pytest, and other frameworks.
Testing
"Just mock it" is the most common piece of testing advice—and one of the most misused. Mocks and stubs both replace real dependencies in tests, but they have fundamentally different purposes. Using one when you need the other leads to tests that pass while hiding real problems.
Jest
You've covered jest.fn() and jest.mock(). Now you're staring at a test that needs to control setTimeout, mock an ES module, or stub a class constructor. These are the patterns where Jest's documentation runs thin and Stack Overflow answers contradict each other. Here&
Shift-Left Testing
Shift-left testing has a culture problem. Teams adopt the tooling — CI pipelines, coverage reporters, test frameworks — but neglect the human side. Six months later, test coverage stagnates, developers treat green pipelines as optional, and the shift-left initiative quietly dies. The teams that succeed with shift-left do something
Godot
GUT (Godot Unit Test) is the de-facto testing framework for Godot projects, and Godot 4 brought significant changes to GDScript that affect how you write and structure tests. If you've gone through the GUT getting-started tutorials and written basic assert_eq tests, this guide covers the
Testing
Robolectric runs Android tests on the JVM — no emulator, no device, no waiting for adb to connect. It implements Android's framework classes as JVM-compatible stubs, so your Activities, Fragments, ViewModels, and Repositories run in a regular JUnit test process. A Robolectric test suite that takes 30 seconds
Visual Testing
Loki is a visual regression testing tool purpose-built for Storybook. It runs locally and in CI, captures screenshots of every story, compares them against stored references, and shows diffs when components change. Unlike cloud-hosted alternatives, Loki stores reference images in your repository and runs entirely offline. Why Loki
Visual Testing
Happo solves a specific problem: visual regression testing across multiple browsers simultaneously. While Chromium-only tools catch 80% of visual regressions, cross-browser differences — font rendering, flexbox quirks, Safari's date inputs — require testing against real browser engines. Happo runs your components in Chrome, Firefox, Safari, and Edge and
Visual Testing
Visual regression testing catches UI changes that unit tests miss. Lost Pixel is an open-source visual regression testing platform that compares screenshots of components or pages across commits, flags differences, and integrates with your existing CI pipeline. Unlike hosted-only tools, Lost Pixel can run entirely in your own
Clojure
re-frame's unidirectional data flow — events → effects → db → subscriptions → views — makes applications predictable. It also makes them testable. Every piece of behavior in a re-frame app is a pure function or a data-driven side effect description. re-frame-test provides the tooling to test them without
Clojure
Example-based tests check specific cases. Property-based tests check invariants across hundreds of randomly generated inputs. test.check is Clojure's property testing library — a port of Haskell's QuickCheck — and it finds bugs that no developer thought to test for. Why Property-Based Testing Consider a
Clojure
Kaocha is the most flexible test runner in the Clojure ecosystem. While lein test and clojure -M:test get you started, Kaocha gives you fine-grained control over test selection, execution order, output formatting, and extensibility through plugins. This guide covers advanced Kaocha configuration for teams that need more than
Clojure
Babashka is a native Clojure scripting runtime that starts in milliseconds. It runs Clojure code without the JVM startup penalty, making it practical for scripts that need to run frequently — build tools, CI helpers, file processors, HTTP clients. But scripts grow complex, and complex code needs tests. This guide covers