爲什麼這些JavaScript函數不會添加到此對象中？

Question

我想爲基於Json文件的對象編寫getter和setter函數。我複製了John Resig的書（Appress Pro JavaScript Techniques）的下面的代碼，但它不起作用，並且這些函數不會添加到對象中。您能否告訴我爲什麼以及什麼是正確的代碼？

// Create a new user object that accepts an object of properties 
function User(properties) { 
// Iterate through the properties of the object, and make sure 
// that it's properly scoped (as discussed previously) 
    for (var i in properties) { 
    (function() { 
     // Create a new getter for the property 
     this["get" + i] = function() { 
      return properties[i]; 
     }; 

     // Create a new setter for the property 
     this["set" + i] = function (val) { 
      properties[i] = val; 
     }; 
    })(); 
    } 
} 

// Create a new user object instance and pass in an object of 
// properties to seed it with 
var user = new User({ 
name: "Bob", 
age: 44 
}); 

// Just note that the name property does not exist, as it's private 
// within the properties object 
//alert(user.name == null); 

// However, we're able to access its value using the new getname() 
// method, that was dynamically generated 
alert(user.getname()); 

Answer 1

你使用的函數來創建一個封閉，但你忘了通過i進去。您還需要在函數內的this的不同參考，因爲上下文在其中更改爲window。

function User(properties) { 
    var i, me = this; 
    for (i in properties) (function (i) { 
     // Create a new getter for the property 
     me["get" + i] = function() { // using `me` because `this !== me` 
      return properties[i]; 
     }; 

     // Create a new setter for the property 
     me["set" + i] = function (val) { 
      properties[i] = val; 
     }; 
    }(i)); // i passed into function closure 
} 

Answer 2

在你IIFE，this實際上是window。您可以通過使用Function.prototype.call指定上下文：

function User(properties) { 
    // Iterate through the properties of the object, and make sure 
    // that it's properly scoped (as discussed previously) 
    for (var i in properties) { 
     (function(i) { 
      // Create a new getter for the property 
      this["get" + i] = function() { 
       return properties[i]; 
      }; 

      // Create a new setter for the property 
      this["set" + i] = function(val) { 
       properties[i] = val; 
      }; 
     }).call(this, i); 
    } 
} 

或者保持對它的引用與另一個變量：

function User(properties) { 
    var that = this; 

    // Iterate through the properties of the object, and make sure 
    // that it's properly scoped (as discussed previously) 
    for (var i in properties) { 
     (function(i) { 
      // Create a new getter for the property 
      that["get" + i] = function() { 
       return properties[i]; 
      }; 

      // Create a new setter for the property 
      that["set" + i] = function(val) { 
       properties[i] = val; 
      }; 
     })(i); 
    } 
} 

如果你是在嚴格模式下，您的代碼將拋出一個錯誤，而不是誤導繼承：

TypeError: Cannot set property 'getname' of undefined