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JavaScript null and undefined — exercises and bug fixes

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

    Report an unassigned variable

    easy10 min

    Report the value of a variable that was declared but not set.

  2. 2

    Report a missing property

    easy10 min

    Read a property that does not exist and report it.

  3. 3

    Empty a value on purpose

    easy10 min

    Set a variable to null and report it.

  4. 4

    Check for either

    easy10 min

    Use one check that catches both null and undefined.

  5. 5

    See what JSON keeps

    medium10 min

    Stringify an object with a null and an undefined property.

Find the bug(5)

  1. 1

    Fix the check that misses undefined

    easy8 min

    Strict equality against null does not catch undefined.

  2. 2

    Fix the zero rejected as missing

    easy8 min

    A truthy check treats a valid zero as absent.

  3. 3

    Fix typeof used to detect null

    easy8 min

    typeof null is object, so it cannot identify null.

  4. 4

    Fix the property set to undefined

    easy8 min

    Setting a property to undefined leaves it in the object but drops it from JSON.

  5. 5

    Fix the wrong emptiness assigned

    easy8 min

    A deliberate emptying should use null, not the text undefined.

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