「這個」在一個匿名函數裏面？

Question

裏面約翰Resig的書「臨的JavaScript技術。」他描述生成與下面的代碼動態對象方法的一種方式：

// 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; 
      }; 
     })(); 
    } 
} 

問題是，當我嘗試實例上述目的，動態方法正在連接到窗口對象而不是實例化的對象。看起來「這個」是指窗口。

// 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 
}); 

alert(user.getname()); 

運行上面的代碼會引發此錯誤「user.getname不是函數」。

爲每個實例化對象生成動態函數的正確方法是什麼？

Answer 1

這是本書的代碼嗎？我有這本書，但我沒有讀過它。

這是書中的錯誤。檢查勘誤：http://www.apress.com/9781590597279

裏面的匿名函數，this是全球window。

你可以把它通過後加入.call(this, i)工作。

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); 
    } 
} 

Answer 2

this中的內部自我執行功能與外部User功能中的此不相同。正如你注意到的，它指的是全球的window。

如果稍有通過添加是指外this可變修改代碼的問題是固定的。

function User(properties) { 
    var self = this; 
    for (var i in properties) { 
    (function() { 
     self["get" + i] = function() { /* ... */ }; 
     self["set" + i] = function() { /* ... */ }; 
    })(); 
    } 
} 

這就是說，我不知道爲什麼連這裏需要匿名自動執行功能，讓你擁有的只是留了簡單的選擇完全，像這樣：

function User(properties) { 
    for (var i in properties) { 
     this["get" + i] = function() { /* ... */ }; 
     this["set" + i] = function() { /* ... */ }; 
    } 
} 

Answer 3

以下是如何操作。您需要將上下文保存到另一個變量中。另一種選擇是不要執行你在for循環中做的這個內部函數。

// 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) 
    var that = this; 
    for (var i in properties) { (function(){ 
     // 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; 
     }; 
    })(); } 
} 

選項2：

// 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) { 
     // 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; 
     }; 
    } 
} 

Answer 4

你總是可以強制另一this任何函數調用，使用apply方法。

(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; 
    }; 
}).apply(this);