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JavaScript Closures — exercises and bug fixes

5 hands-on exercises and 5 find-the-bug challenges for the JavaScript Closures 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

    Build a counter

    easy10 min

    Build a counter with a closure and call it three times, joining the results.

  2. 2

    Keep a value private

    easy10 min

    Store a balance in a closure, deposit 50 onto 100, and show the balance.

  3. 3

    Build a function factory

    easy10 min

    Make a multiplier that triples, then use it on 14.

  4. 4

    Separate closures

    easy10 min

    Show that two counters do not share their count, joined by a comma.

  5. 5

    Remember whether it ran

    medium10 min

    Use a closure so a setup function only ever returns its first answer.

Find the bug(5)

  1. 1

    Fix the counter that resets

    easy8 min

    The count is declared inside the returned function, so it starts again every call.

  2. 2

    Fix the balance that is not kept

    easy8 min

    The deposit works on a copy, so the balance never grows.

  3. 3

    Fix the factory that returns a number

    easy8 min

    The outer function should return a function, not a value.

  4. 4

    Fix the shared count

    easy8 min

    The count sits outside the factory, so both counters share it.

  5. 5

    Fix the loop closure

    easy8 min

    With var, all three functions share one variable.

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