create a simple and fast memoization function for calculation.

memoization.js

/*
use this memoiziation function with immutable js and see the 
beautifulness of Js 
calculate one time and use it every where
*/

function memoize(fn) {
	let prevArg;
	let prevResult;
	return function (arg) {
		return arg === prevArg 
			?	prevResult 
			:   (prevArg = arg, prevResult = fn.call(this, arg));
	}
}

function sum(list) {
	return list.reduce((a, b) => a + b);
}

let memoSum = memoize(sum);

let listOfOneBilionNumber = [...Array(1000000).keys()];

console.time("sum");
console.log(memoSum(listOfOneBilionNumber));
console.timeEnd("sum");

console.time("sum");
console.log(memoSum(listOfOneBilionNumber));
console.timeEnd("sum");

listOfOneBilionNumber.push(1001);

console.time("sum");
console.log(memoSum(listOfOneBilionNumber));
console.timeEnd("sum");

/* 
because of we comapare arg === prevArg so when we add new item 
to the array so the comparison will no sense to this change 
so it return true and the function will not work correctly
*/

/*
import {List} from "immutable";
let list = List(listOfOneBilionNumbers)
*/

A note about immutable structure data in + immutable.js