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JavaScript WeakMap and WeakSet — exercises and bug fixes

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

    Store against an object

    easy10 min

    Use an object as a WeakMap key and read the value back.

  2. 2

    Check for a key

    easy10 min

    Report whether the object is in the WeakMap.

  3. 3

    Reject a string key

    easy10 min

    Catch the error when a string is used as a key.

  4. 4

    Show there is no size

    easy10 min

    Report that a WeakMap has no size property.

  5. 5

    Use a WeakSet

    medium10 min

    Add an object and check a different one is not present.

Find the bug(5)

  1. 1

    Fix the primitive key

    easy8 min

    A WeakMap key must be an object.

  2. 2

    Fix the attempt to loop

    easy8 min

    A WeakMap is not iterable, so it cannot be spread.

  3. 3

    Fix the new object used as a key

    easy8 min

    A fresh object is a different key, so nothing is found.

  4. 4

    Fix WeakSet used with a primitive

    easy8 min

    A WeakSet holds objects only.

  5. 5

    Fix the Map chosen where a WeakMap fits

    easy8 min

    A normal Map keeps its keys alive, which is the leak being avoided.

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