Extends the Math object with isPrime, optimized to check only odd numbers greater than two and less than square root.

Math.isPrime

Math.isPrime = function (n) {
    if (n === 2) { return true; }
    if (n % 2 === 0) { return false; }
    for(let i = 3, s = Math.sqrt(n); i <= s; i += 2) {
        if (n % i === 0) { return false; }
    }
    return n !== 1;
};

// Based on this StackOverflow answer: https://stackoverflow.com/a/40200710/2502532
// JSPerf test to demonstrate optimization: https://jsperf.com/math-isprime/1

My ReasonML experimentation this morning at the coffee shop:

/* inspired by https://gist.github.com/geraldfullam/164ce3268ebc896c0cf06e82220da409 */

let rec checkDivisibility = (y, limit, x) => {
  if(y >= limit) {
    false
  } else if (x mod y === 0) {
    true 
  } else {
    checkDivisibility(y + 2, limit, x)
  }
};

let isPrime = (x: int): bool => {
  if (x === 2) {
    true 
  } else if(x mod 2 === 0) {
    false
  } else if (checkDivisibility(3, int_of_float(sqrt(float_of_int(x))), x)) {
    false 
  } else {
    x !== 1 
  }
};

Js.log(isPrime(1));
Js.log(isPrime(2));
Js.log(isPrime(3));
Js.log(isPrime(123));
Js.log(isPrime(1234));
Js.log(isPrime(12345));
Js.log(isPrime(12346));
Js.log(isPrime(12347));
Js.log(isPrime(123478));
Js.log(isPrime(123478));
Js.log(isPrime(1234789));
Js.log(isPrime(12347890));