Kata

Let's assume we have a slow pure function.

function double(x) {
  for(let i = 0; i < 1e9; i++);
  return x * 2;
}

double(10) === 20 // but it takes ≈1s

Your task is to implement the memo decorator. This function accepts one argument fn and returns a new fuction which does the same work but doesn't compute a result for the same arguments twice.

const mDouble = memo(double);

mDouble(1) === 2 // ≈1s
mDouble(1) === 2 // ≈0.001s
mDouble(3) === 6 // ≈1s
mDouble(1) === 2 // ≈0.001s
mDouble(3) === 6 // ≈0.001s

The fn should work with any type and any length of function arguments. For example, the arguments can be numbers, bigints, arrays, objects or functions.

Similar katas:

• Reusable memoisation — a decorated function is a function of one argument
• Function Cache — a very similar kata, but it has weaker tests