function power_of_2(n) {  
 if (typeof n !== 'number')   
      return 'Not a number';   
  
    return n && (n & (n - 1)) === 0;  
}  
  
console.log(power_of_2(16));  
console.log(power_of_2(18));  
console.log(power_of_2(256));