有没有一种方法可以避免在此JavaScript代码块中进行评估？

Question

Is there a way to avoid eval in this block of js?

// 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

The eval here is completely pointless:

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

This will do the exact same thing, also note that r leaks into the global scope.

// 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');
 }