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# How to check if a number is prime in JavaScript

The following function returns true if the parameter n is a prime number:

// Return true if n is prime
function isPrime(n)
{
// Check if n is less than 2 => not prime
if (n<2) return false;

// Loop from 2 to square root of n
for (let i = 2; i <= Math.sqrt(n); i++)
// If i is a divisor of n, n is not prime
if (n % i == 0) return false;

return true;
}

This function check every divison between 2 and square root of n. If n is not prime, there is necessarily a divisor in this interval.