last =  function(array, n) {  
  if (array == null)   
    return void 0;  
  if (n == null)   
     return array[array.length - 1];  
  return array.slice(Math.max(array.length - n, 0));    
  };  
  
console.log(last([7, 9, 0, -2]));  
console.log(last([7, 9, 0, -2],3));  
console.log(last([7, 9, 0, -2],6));