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JavaScript setInterval — exercises and bug fixes

5 hands-on exercises and 5 find-the-bug challenges for the JavaScript setInterval lesson. Every one is checked automatically in the browser — write the code, run it, and the tests tell you what still fails.

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Exercises(5)

  1. 1

    Tick three times

    easy10 min

    Use setInterval to count to three, then stop and report the count.

  2. 2

    Count down

    easy10 min

    Count down from three and report liftoff at the end.

  3. 3

    Stop after one run

    easy10 min

    Clear the interval inside its own first run.

  4. 4

    Build a total

    easy10 min

    Add one each tick until the total reaches five, then report it.

  5. 5

    Repeat with setTimeout instead

    medium10 min

    Use a repeating setTimeout so each run schedules the next.

Find the bug(5)

  1. 1

    Fix the interval that never stops

    easy8 min

    Without clearInterval the count keeps rising past three.

  2. 2

    Fix the id not kept

    easy8 min

    clearInterval needs the id that setInterval returned.

  3. 3

    Fix setTimeout used where a repeat is needed

    easy8 min

    setTimeout runs once, so the count never reaches three.

  4. 4

    Fix the countdown ending early

    easy8 min

    The interval stops one step too soon.

  5. 5

    Fix the interval cleared before it runs

    easy8 min

    Clearing it straight away means the callback never fires at all.

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