有没有办法在这个JavaScript块中避免使用eval？

Question

有没有办法在这块js中避免使用eval？

// Check requirements Prototype and Scriptaculous
(function () {
 var requires = [
  'Prototype'
  , 'Scriptaculous'
 ]
 while (r = requires.pop()) {
  if (eval('typeof ' + r + ' == "undefined"')) alert(r + ' is required');
 }
} ());

Answer 1

eval这里完全没有意义：

// since everything in the global scope gets defined on 'window'
typeof window[r] === 'undefined'; 

这将执行完全相同的操作，同时请注意r泄漏到全局范围。

// Check requirements Prototype and Scriptaculous
(function () {
    var requires = ['Prototype', 'Scriptaculous'];

   var r = ''; // make sure to __not__ override a global r
   while (r = requires.pop()) {
       if (typeof window[r] === 'undefined') {
           alert(r + ' is required');
       }
   }
} ());

Answer 2

怎么样

if (typeof window[r] == "undefined")

Answer 3

 while (r = requires.pop()) {
  if (typeof window[r] == "undefined") alert(r + ' is required');
 }