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JavaScript Memoization — exercises and bug fixes

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

    Cache a function's result

    easy10 min

    Memoize a function and confirm it only really runs once.

  2. 2

    Return the cached value

    easy10 min

    Confirm the memoized function still returns the right answer.

  3. 3

    Cache different inputs separately

    easy10 min

    Call with two different inputs and count the real calls.

  4. 4

    Speed up recursive Fibonacci

    easy10 min

    Memoize a recursive Fibonacci and get a value that would be slow otherwise.

  5. 5

    Build a key from two arguments

    medium10 min

    Memoize a two-argument function using a combined key.

Find the bug(5)

  1. 1

    Fix the cache never checked

    easy8 min

    Without checking the cache first, every call runs the real function.

  2. 2

    Fix the cache declared inside the wrapper

    easy8 min

    A new cache on every call means nothing is ever remembered.

  3. 3

    Fix the two arguments sharing one key

    easy8 min

    Different inputs must not overwrite each other in the cache.

  4. 4

    Fix an impure function memoized

    easy8 min

    Memoizing Math.random always returns the very first value forever.

  5. 5

    Fix the result stored before computing it

    easy8 min

    The value must be computed before it can be cached.

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