Wallaby.js

About Wallaby.js

Wallaby.js is a test runner that executes JavaScript and TypeScript tests as code is typed, displaying results, code coverage, and runtime values directly in the code editor. It integrates as a plugin into existing testing frameworks (such as Jest) and IDEs (such as VS Code) rather than replacing them, and it recalculates and runs only the minimum set of tests affected by a given code change. It supports React applications using Jest in VS Code, and includes a time travel debugger for stepping forward and backward through code execution, along with inline error reporting, inline coverage indicators, diff and snapshot management, a test execution viewer, and a performance profiler.

The tool also provides a Skill and MCP server that give AI coding assistants live access to test results, runtime values, execution paths, coverage, and dependency graphs, and it is described as working with Copilot Agent, Codex, Claude Code, Cursor, and Windsurf.

It is intended for developers writing automated tests in JavaScript or TypeScript who want real-time feedback during editing rather than running a full test suite separately, and for teams using AI coding assistants that benefit from runtime test context.

Key features

• Runs JavaScript/TypeScript tests instantly as you type, streaming results next to your code
• Time Travel Debugger to step forward/backward through code execution and view runtime values
• Inline code coverage indicators updated in real time in the editor gutter
• MCP server and Skill giving AI agents live access to test results, runtime values, coverage, and dependencies
• Runs only the minimum set of tests affected by a code change for faster feedback
• Test Profiler for recording and analyzing a test's CPU usage

Frequently asked questions