https://projecteuler.net/problem=3

project_euler_example_3.js

function isPrime(number) { 
  var index;
  var limit = Math.ceil(Math.sqrt(number));
  // since the main loop generates odd numbers only
  // we can start testing primality dividing by 3
  for (index = 3; index <= limit; index += 2) {
      if (number % index === 0) {
          return false;
      }
  }
  return true;
}

function largestPrimeFactor(number) {
    var factor = Math.ceil(Math.sqrt(number));

    // start with an odd number
    factor = (factor & 1) === 0 ? factor - 1 : factor;

    while (!(number % factor === 0 && isPrime(factor))) {
        factor -= 2; // odd numbers only
    }
    return factor;
}
largestPrimeFactor(600851475143);