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JavaScript Proxy — exercises and bug fixes

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

    Intercept a read

    easy10 min

    Create a Proxy that passes reads straight through.

  2. 2

    Log every read

    easy10 min

    Record which keys were read through the proxy.

  3. 3

    Validate a write

    easy10 min

    Reject a negative age through a set trap.

  4. 4

    Supply a default for a missing key

    easy10 min

    Return a fallback string for any property not on the object.

  5. 5

    Use the has trap

    medium10 min

    Make one extra key always report as present.

Find the bug(5)

  1. 1

    Fix the set trap that forgets to store the value

    easy8 min

    The trap must actually assign the value on the target.

  2. 2

    Fix the set trap missing its return value

    easy8 min

    A set trap must return true, or the assignment is reported as failed even though the value was stored.

  3. 3

    Fix the validation checking the wrong key

    easy8 min

    The validation compares against the wrong property name.

  4. 4

    Fix the direct target read counted as a proxy read

    easy8 min

    Reading the original target bypasses the trap entirely, so nothing is counted.

  5. 5

    Fix the has trap that ignores the real object

    easy8 min

    Any key not specially handled should fall back to checking the object.

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