function product_Range(a,b) {  
      var prd = a,i = a;  
       
      while (i++< b) {  
        prd*=i;  
      }  
      return prd;  
    }  
      
      
    function combinations(n, r)   
    {  
      if (n==r)   
      {  
        return 1;  
      }   
      else   
      {  
        r=(r < n-r) ? n-r : r;  
        return product_Range(r+1, n)/product_Range(1,n-r);  
      }  
    }  
      
      
    console.log(combinations(6, 2));  
    console.log(combinations(5, 3));