If it's a tool: for me it is probably Wallaby.js - realtime as you type test runner with super powerful in editor feedback and debugging tools (and as someone else said, Quokka.js its sibling).
Wallaby is the best. Not only is it a joy to use, if you by accident throw a project at it that doesn't work their support team is always very quick to help you out (Example: github.com/wallabyjs/public/issues...).
For other languages there are also alternatives. C# -> nCrunch, Python -> PyCrunch, Java -> InfiniTest.
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