JSFiddle - React, Tailwind, and code Playground

JavaScript

var arr = [1, 2, [3, 4, [[[5]]]]];
num = 0;

function loop (arr) {
    for (var i = 0; i < arr.length; i++) {
        if (arr[i] instanceof Array) {
            loop(arr[i]);
        } else {
            num += parseInt(arr[i]);
        }
    }
}

loop(arr);

console.log(num);