e3 (inefficient)

var number = 600851475143 ;
var currNum = 0;

for(var i = 0; i < Math.ceil(Math.sqrt(number)) ; i ++){

    if(isPrime(i) && i > currNum && number%i == 0){

        var j = number/i ;
        if( j > i && i != 1 && isPrime(j)) {
            currNum = j;
        }else{
            currNum = i;
        }

    }

}

console.log(currNum);