-code With Mosh- Mastering Javascript Unit Testing May 2026

Leo turned to Sarah. "I broke the code on purpose. The tests found it in 0.3 seconds."

Leo paused the video. He looked at his own checkout.js file—a 500-line monster with nested conditionals, global variables, and functions that did seven things at once. No wonder it broke. -Code With Mosh- Mastering JavaScript Unit Testing

He wrote his first failing test:

Leo had been a JavaScript developer for three years. He could spin up a React component in his sleep and chain promises like a poet. Yet, every Friday evening, the same dread washed over him as he typed npm run build . Leo turned to Sarah