In this lesson we develop a prime number algorithm in JavaScript.

First of all, remember that a number is prime when it has two divisors, namely 1 and itself.

The sequence of prime numbers begins with 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, …

## prime number JavaScript – algorithm

Check if a number taken as input is prime.

To check if a number is prime, you need to use a divisor which is incremented by 1 from time to time.

So, I begin to divide the number taken as input, first by 1, then by 2, then by 3, etc.

Clearly in order to do this I have to use a loop, increasing the divisor by 1, after each iteration.

So if, for example, the number were 3 then I would first divide it by 1, then by 2 and then by 3. At the same time I can count the divisors using a special variable, for example named * count*.

If at the end we find 2 divisors, that is 1 and the number itself, the value of the variable * count* will be

**2**and then

**the number is prime**.

**Otherwise**the number

**is not prime**.

But we can do better. We can consider that the divisor can stop at half of the number itself as it is taken for granted that dividing a number by a value greater than its half you get a decimal number. So we can only count one divisor to define that a number is prime (if count wille be 1 the number is prime).

So here is a possible solution to the algorithm on prime number created in JavaScript:

```
var n = prompt('Insert a number: ');
while (n < 0){
n = prompt('Insert a positive number: ');
}
var div = 1;
var count = 0;
while(count <= 1 && div <= n/2) {
if(n % div == 0) {
count++;
}
div++;
}
if (count == 1){
document.write('The number is prime!');
}
else {
document.write('The number isn't prime!');
}
```

In this algorithm we first checked that the user enters a positive number with a while loop, in the same way we could also use the do while.

The while condition we entered stops as soon as the variable count equals 1 and the divisor has reached half the number.

## Conclusion

Clearly this is just a possible solution to the prime number algorithm in JavaScript, please submit yours and let's discuss it in the comments below.

### Some useful links

Introduction to JavaScript language

Learn JavaScript – basic concepts