determine whether a number is prime

prime.js

if (process.argv.length !== 3) {
        console.log("Usage:", process.argv[1], "num")
        return;
}

var primeNum = parseInt(process.argv[2]);
if (isNaN(primeNum)) {
        console.log("is not even a number, let alone prime");
        return;
}

for (var i = 2; i < primeNum; ++i) {
        if (primeNum % i === 0) {
                console.log(primeNum + " is not prime");
                return;
        }
}

console.log(primeNum + " is prime");