This is checked so that we can skip middle five numbers in below loop
if ( n % 2 == 0 n % 3 == 0 ) { return false ; } for ( let i = 5 ; i * i <= n ; i = i + 6 ) { if ( n % i == 0 || n % ( i + 2 ) == 0 ) { return false ; } } return true ; }