underscore tutorial - Prime Numbers

simple underscore.js projects, walk through, & tutorial

findNextPrime(), computing prime numbers

Basic Prime Number Generator:

function findNextPrime(x){

    // no test conditions, hardwired
    if (x < 0){return 0;}
    if (x < 1){return 1;}
    if (x < 2){return 2;}
    if (x < 3){return 3;}

    var isPrime = true;

    var divisor = 2;

    // This is the maximum divisor value that needs to be checked
    // As any higher divisor will be matched by some lower one
    // which would have already been checked for
    var maxDivisor = Math.floor(Math.sqrt(x) + 2);

    x = Math.floor(x);

    var nextTest = x + 1;

    while (divisor <= maxDivisor){

        if (nextTest%divisor === 0){
        isPrime = false;
        }

        divisor += 1;

    } // if nextTest / divisor then it's not a prime

    if (isPrime === true){
        return nextTest;
    } else {
        return findNextPrime(nextTest);
    } // interator that feed findNextPrime into itself

} // end findNextPrime();

document.getElementById('outputOne').innerHTML = findNextPrime(9);

Output:

Nada Yet

Find the the ten smallest prime numbers:

outputTwo[0] = 0;
for (i = 1; i <= 10; i++){
outputTwo[i] = findNextPrime(outputTwo[i - 1]);
}

Output:

Nada Yet

A Web Search indicated these where the three smallest six digit prime numbers:
100003, 100019, 100043, 100049

So for a final test, let's see if those numbers are what the program kicks out:

outputThree = findNextPrime(100000) + ', ' + findNextPrime(100004) + ', ' + findNextPrime(100020) + ', ' + findNextPrime(100045);

Output:

Nada Yet