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09

Loops and Common Errors: Catch Broken Repetition Early

Learn to diagnose infinite loops, wrong limits and counters moving in the wrong direction by observing state step by step instead of changing code blindly.

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📘 Theory

Why So Many Loops Break at the Beginning

A loop is only safe when the condition and the changing value work together.

A loop can fail even when the syntax is valid. The real issue is usually logic: a value never changes, changes too far, or changes in the wrong direction.

That is why debugging loops is mostly about reading state, not about rewriting everything from scratch.

The condition is wrong.
The control value never updates.
The value changes, but away from the exit condition.
You are not checking what really happens during each iteration.

Error 1: Infinite Loop Because the Control Value Never Changes

If the loop condition stays true forever, the loop never escapes.

Error 2: Updating in the Wrong Direction

The counter may change on each turn and still never get closer to the exit.

1 Why So Many Loops Break at the Beginning 2 Error 1: Infinite Loop Because the Control Value Never Changes 3 Error 2: Updating in the Wrong Direction

🧪 Learn by doing

Example Guided Example: Fix an Infinite While Loop Add the missing update so the loop advances and stops.

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🏁 Challenges

Challenge Challenge: Repair the Broken Counter Correct a loop that moves in the wrong direction.

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🧰 Resources

Useful links

MDN: Loops and iteration MDN: Debugging JavaScript Chrome DevTools: JavaScript debugging reference

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Course resources 15

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Loops and Repetition: Avoid Repeated Work with `for` and `while`

Functions, Blocks, and Return: Reuse Logic Without Copying Code