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JavaScript Async Callbacks — exercises and bug fixes

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

    Pass a callback in

    easy10 min

    Write a loader that hands its result to a callback.

  2. 2

    Use the error-first pattern

    easy10 min

    Call back with null for the error and a name for the value.

  3. 3

    Report a failure

    easy10 min

    Call back with an error and report it.

  4. 4

    Nest two steps

    easy10 min

    Run one callback inside another and report both results.

  5. 5

    Use a named handler

    medium10 min

    Pass a named function as the callback rather than an inline one.

Find the bug(5)

  1. 1

    Fix the result returned instead of passed

    easy8 min

    Returning from inside a timer gives the caller nothing.

  2. 2

    Fix the arguments in the wrong order

    easy8 min

    The error comes first and the value second.

  3. 3

    Fix the error never checked

    easy8 min

    The failure is being reported as a success.

  4. 4

    Fix the callback called with brackets

    easy8 min

    Passing callback() hands over its result, not the function.

  5. 5

    Fix the result used outside the callback

    easy8 min

    The second half is added before the callback has supplied the first.

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