JSFiddle - React, Tailwind, and code Playground

JavaScript

var array = [];
    array[0] = 1;
    array[2] = 2;
    array.hello_world = 3;

    var a = 0;
    for(var i = 0; i<array.length; i++){
        a += array[i];
    }
    console.log("index: " + a);

    var b = 0;
    array.forEach(function (v, i) {
        b += v;
    });
    console.log("forEach: " + b);

    var c = 0;
    for (var p in array) {
        c += array[p];
    }
    console.log("for..in: " + c);