isprime.js

var isInteger = function(x) { return (x ^ 0) === x; };

function isPrime(number) {
	if (typeof number !== 'number' || !isInteger(number)) {
		// Alternatively you can throw an error.
		return false;
	}

	if (number < 2) {
		return false;
	}

	if (number === 2) {
		return true;
	} else if (number % 2 === 0) {
		return false;
	}

	var squareRoot = Math.sqrt(number);
	for(var i = 3; i <= squareRoot; i += 2) {
		if (number % i === 0) {
			return false;
		}
	}

	return true;
}

console.log('isPrime(5):', isPrime(5));
console.log('isPrime("10"):', isPrime("10"));