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JavaScript Iterators — exercises and bug fixes

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

    Call next manually

    easy10 min

    Get an array's iterator and read its first value.

  2. 2

    Check the done flag

    easy10 min

    Drain a one-element array and check the second call is done.

  3. 3

    Write a custom iterator

    easy10 min

    Build an iterator object that counts to three.

  4. 4

    Drain an iterator by hand

    easy10 min

    Collect every value from a custom iterator into an array.

  5. 5

    Check for the iterator method

    medium10 min

    Report whether a plain object has Symbol.iterator.

Find the bug(5)

  1. 1

    Fix the value read as the whole result

    easy8 min

    next() returns an object; the value is one property of it.

  2. 2

    Fix the loop that never stops

    easy8 min

    Without checking done, the loop keeps calling next past the end.

  3. 3

    Fix the custom iterator returning a value with done true

    easy8 min

    Once finished, value should be undefined, not a leftover number.

  4. 4

    Fix the plain object assumed to be iterable

    easy8 min

    A plain object has no Symbol.iterator method, so spreading it throws.

  5. 5

    Fix done checked with the wrong value

    easy8 min

    done is a boolean, not the string "true".

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