有沒有一種方法可以避免在這個JavaScript塊中使用eval？

Question

有沒有辦法在這個js塊中避免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

這裏的eval是完全沒有意義的：

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

這將做確切同樣的事情，也注意到r泄漏到了全球範圍。

// 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

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

Answer 3

如何

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