JSFiddle - React, Tailwind, and code Playground

JavaScript

var numbers = [5, 7, 20, 3, 13];

var arr1 = [];
var len = numbers.length;
for(var i = 0; i < len; i++){
    arr1.push(numbers[i] * 2);
}
alert(arr1);

var arr2 = [];
for(var i = 0; i < len; i++){
    arr2[arr2.length] = numbers[i] * 2;
}
alert(arr2);