Question

What is the output?

const add = () => {
  const cache = {};
  return num => {
    if (num in cache) {
      return `From cache! ${cache[num]}`;
    } else {
      const result = num + 10;
      cache[num] = result;
      return `Calculated! ${result}`;
    }
  };
};

const addFunction = add();
console.log(addFunction(10));
console.log(addFunction(10));
console.log(addFunction(5 * 2));

• A: Calculated! 20 Calculated! 20 Calculated! 20
• B: Calculated! 20 From cache! 20 Calculated! 20
• C: Calculated! 20 From cache! 20 From cache! 20
• D: Calculated! 20 From cache! 20 Error

Answer 1

Answer: C

The add function is a memoized function. With memoization, we can cache the results of a function in order to speed up its execution. In this case, we create a cache object that stores the previously returned values.

If we call the addFunction function again with the same argument, it first checks whether it has already gotten that value in its cache. If that's the case, the caches value will be returned, which saves on execution time. Else, if it's not cached, it will calculate the value and store it afterwards.

We call the addFunction function three times with the same value: on the first invocation, the value of the function when num is equal to 10 isn't cached yet. The condition of the if-statement num in cache returns false, and the else block gets executed: Calculated! 20 gets logged, and the value of the result gets added to the cache object. cache now looks like { 10: 20 }.

The second time, the cache object contains the value that gets returned for 10. The condition of the if-statement num in cache returns true, and 'From cache! 20' gets logged.

The third time, we pass 5 * 2 to the function which gets evaluated to 10. The cache object contains the value that gets returned for 10. The condition of the if-statement num in cache returns true, and 'From cache! 20' gets logged.