JSFiddle - React, Tailwind, and code Playground

JavaScript

var numberInQuestion = prompt("Of what number are you wanting to find the largest        prime factor?");

function factors(numberInQuestion){
    var factorsArray = [];
    for(var i = 2; i < numberInQuestion-1; i++){
        if(numberInQuestion % i === 0 && isPrime(i)){
            factorsArray.push(i);
        }
    }
    return factorsArray; 
};
var factors = factors(numberInQuestion);
alert(factors[factors.length-1]);

//From: http://stackoverflow.com/questions/11966520/how-to-find-prime-numbers
function isPrime (n)
{
    if (n < 2) return false;
    var q = Math.sqrt (n);

    for (var i = 2; i <= q; i++)
    {
        if (n % i == 0)
        {
            return false;
        }
    }

    return true;
}