Best Code to check if number is Prime

this doesn't work for all numbers
tested with 1111111 & 121111

[EDIT1]replace the loop by

for(int i = 5 ; i*i <= n; i = i + 4){
        if(n % i == 0 || n % (i+2) == 0) return false;
    }
    return true;

[EDIT2] to explain : the return true in the for loop will always return true on the first try, and the && in the if need to be a ||
to test if it works, take high odd number, and if you wanna be sure, add printf("%d %% %d = %d | ", n, i, n%i); and printf("%d %% %d = %d | ", n, i+2, n%(i+2)); in the for loop (that's a fastest way instead of debug)

In JS, using regex:

const isPrime = x=>!'1'.repeat(x).match(/^1?$|^(11+?)\1+$/)