Spoiler Alert: Project Euler 3

euler3.js

var number = 600851475143;

var isPrime = function(num) {
  var i = Math.ceil(Math.sqrt(num)),
      end = Math.ceil(num / 2);

  for(; i < end; i++) {
    if (num % i === 0) return false;
  }

  return true;
}


var largestPrimeFactor = function(n) {
  var i = 2,
      origN = n;

  for (; i < n; i++) {
    if (isPrime(i) && n % i === 0) {
      if ((number /= i) === 1) {
        return i;
      }
    }
  }

  return origN;
}

console.log(largestPrimeFactor(number));