Find the lost item in an ordered array. All items are integer and all numbers between 1 to n. Only one is lost, find the lost one.

find_the_lost_item_in_an_ordered_array.js

// practical way
let arr = [1,7,4,2,8,9,3,6];
let arrLength = arr.length;
let sum2=0;
arr.forEach(item=>{sum2+=item});
console.log(((arrLength+1)*(arrLength+2)/2)-sum2)