memoization function in javascript

Question

So I came across a momoize function when I was trying to understand how memoization really works, the solution has really had me thinking, the code below

const memoize = (fn) => {
  const cache = {};
  return (...args) => {
    let cacheKey = args.map(n => n.toString() + '+').join('');
    if (cacheKey in cache) {
      console.log(cache[cacheKey])
      return cache[cacheKey];
    }else {
      let result = args.reduce((acc, curr) => fn(acc, curr), 0);
      cache[cacheKey] = result;
      console.log(result)
      return result;
    }
  }
}
const add = (a, b) => a + b;
const memoizeAdd = memoize(add)
memoizeAdd(1, 2, 3, 4)

My question is how the momoize variable takes the add function as an argument and memoizeAdd also takes a wide range of argument, If add function only accepts 2 arguments? please, this question comes from a place of curiosity.

jkoestinger · Accepted Answer · 2022-04-19 10:03:01Z

1

add is passed as the argument to the memoize function. If you look closely, you will notice that fn (which is referring to add) is always called with two arguments only.

This is because the memoize function calls the fn argument along with reduce (which is meant to process an array, even without a defined length).

Effectively, applying reduce with a function that just adds two parameters will return the sum of all the elements, which is the expected result of memoizeAdd.

I think checking how reduce works might help you

answered Apr 19, 2022 at 10:03

jkoestinger

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1 Comment

Marshall Over a year ago

Thanks, I get it now. I issue was the fact the the add function was first pass as an argument, before memoizeAdd took the second argument. Now I full understand, knowing that the add function is used as the callback for the reduce function.

harpal · Accepted Answer · 2022-04-19 10:22:23Z

Two things are there.

We can pass function as parameter in JavaScript(and can be executed from there).
And what you have been created is a closure

A closure is the combination of a function bundled together (enclosed) with references to its surrounding state (the lexical environment).

When you created and assigned memoizeAdd, closure creates its scope for add function. and returns another function that takes many(...args) arguments.

now you called that returned method(memoizeAdd) with arguments.

the variable chache will be available for this add method scope.

Now if you creates another memoize

const mul = (a, b) => a + b;
const memoizeMul = memoize(add)
memoizeMul(1, 2, 3, 4)

Now it creates another scope and keep cache separate from the add version.

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