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JavaScript Custom Iterables — exercises and bug fixes

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

    Make an object iterable

    easy10 min

    Add a Symbol.iterator generator to a range object.

  2. 2

    Make a class iterable

    easy10 min

    Add [Symbol.iterator] to a class using generator method syntax.

  3. 3

    Transform values while iterating

    easy10 min

    Yield an uppercased version of each item.

  4. 4

    Write an iterable without a generator

    easy10 min

    Return a plain iterator object from Symbol.iterator.

  5. 5

    Destructure a custom iterable

    medium10 min

    Pull two values out of a custom iterable object.

Find the bug(5)

  1. 1

    Fix the missing brackets around Symbol.iterator

    easy8 min

    Symbol.iterator is a computed key and needs square brackets.

  2. 2

    Fix this lost inside the generator

    easy8 min

    Neither an arrow function nor a plain function called without a receiver keeps this bound to the object.

  3. 3

    Fix the class generator method missing its star

    easy8 min

    Without the star, this is not a generator and cannot yield.

  4. 4

    Fix the manual iterator that never finishes

    easy8 min

    Without a done: true branch, the iterator keeps handing out results past the end of the range.

  5. 5

    Fix the destructuring reading properties instead of iterating

    easy8 min

    Destructuring a custom iterable uses its iterator, not its property names — which here are not even called from and to.

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