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JavaScript Private Fields — exercises and bug fixes

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

    Declare a private field

    easy10 min

    Keep the balance private and expose it through a method.

  2. 2

    Hide it from Object.keys

    easy10 min

    Report the keys of an object with one private and one public field.

  3. 3

    Control how it changes

    easy10 min

    Refuse a negative deposit and report the balance.

  4. 4

    Write a private method

    easy10 min

    Keep the tax calculation private and use it from total.

  5. 5

    Expose it through a getter

    medium10 min

    Give the class a balance getter that reads the private field.

Find the bug(5)

  1. 1

    Fix the underscore convention

    easy8 min

    An underscore does not actually prevent anything.

  2. 2

    Fix the private field exposed

    easy8 min

    The field is public, so it shows up in Object.keys.

  3. 3

    Fix the missing validation

    easy8 min

    A negative deposit is being accepted.

  4. 4

    Fix the helper left public

    easy8 min

    The tax helper should not be callable from outside.

  5. 5

    Fix the private field read from outside

    easy8 min

    A private field can only be reached from inside the class body.

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