function nthroot(x, n)  
       {  
        ng = n % 2;  
        if((ng == 1) || x<0)  
           x = -x;  
        var r = Math.pow(x, 1 / n);  
        n = Math.pow(r, n);  
        
        if(Math.abs(x - n) < 1 && (x > 0 === n > 0))  
          return ng ? -r : r;   
       }  
    console.log(nthroot(64, 2));  
    console.log(nthroot(64, -2));