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Debugger

find it · fix it · understand it

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Read working code.
Then repair broken code.

Pick a language. Each one walks you through ten short lessons — real code on the left, the live result on the right. Every lesson ends with the option to debug a program that is broken in exactly the way you just learned about, often with more than one bug hiding in it. Your job is to find where the damage starts and fix it. You get hints when you are stuck. You never get the answer.

How the repair jobs are graded. Nothing checks whether your code matches some "correct" version — there usually is more than one right fix. Each job runs your code for real and tests the result: the page the browser actually built, or the output your program actually printed. If it behaves correctly, it passes.