JavaScript prototype to find co-primes

Co-primes are numbers whose common factor is only 1. For example :- 4, 9

Factors of 4 :- 2, 2, 1
Factors of 9 :- 3, 3, 1
Common Factor :- 1

Here is the JavaScript prototype to find co-primes. Here is the code snippet.

Code (Run Code):

function factors(num) {

arr = [];

for (var i = 1; i < num; i++) {

if (num % i == 0) {

arr.push(i);

}

}

return arr;

}

function coPrime(num1, num2) {

arr1 = factors(num1);

arr2 = factors(num2);

for (var i = 1; i < arr1.length; i++) {

for (var j = 0; j < arr2.length; j++) {

if (arr1[i] == arr2[j]) {

return num1 + ' and ' + num2 + ' are not co-prime';

}

}

}

return num1 + ' and ' + num2 + ' are co-prime';

}

function main() {

document.getElementById('result').innerHTML =

(coPrime(document.getElementById('num1').value, document.getElementById('num2').value))

}