JavaScript 对象内的私有变量定义

Question

我正在学习使用原型属性在 JavaScript 上定义“类类”函数。 当我像这样定义 People 类时，一切都很好：

var People = function(firstName, lastName){
    // public
    this.firstName = firstName;
    this.lastName  = lastName;

    this.fullName = function(){
        console.log(full_name);
        return this;
    };

    // private
    var self = this;
    var full_name = function(){
        return self.firstName + ' ' + self.lastName;
    }();
};

var person = new People('John', 'Smith');
person.fullName();  // => "John Smith"

但是，当我将 fullName 方法移到初始定义之外时，如下所示：

var People = function(firstName, lastName){
    // public
    this.firstName = firstName;
    this.lastName  = lastName;

    // private
    var self = this;
    var full_name = function(){
        return self.firstName + ' ' + self.lastName;
    }();
};

People.prototype.fullName = function(){
    console.log(full_name);
    return this;
};

var person = new People('John', 'Smith');
person.fullName();  // => "ReferenceError: full_name is not defined"

我遇到了一个错误。我不明白为什么...？？

Answer 1

这里：

var full_name = function(){
    return self.firstName + ' ' + self.lastName;
}();

您将变量“full_name”设置为调用该匿名函数的结果。但是，它只是在该构造函数的上下文中声明的变量，所以这并不重要；构造函数之外的任何东西都不能访问它。

因此，在该原型函数中，您会收到一个错误，因为“full_name”实际上没有定义。如果你想要原型上的全名函数，只需在此处声明即可：

People.prototype.fullName = function() {
  return this.firstName + " " + this.lastName;
};

如果您想要一个只能由构造函数中的代码访问的“私有”函数，那么您可以这样做：

var People = function(firstName, lastName){
    // public
    this.firstName = firstName;
    this.lastName  = lastName;

    // private
    var self = this;
    function fullName(){
        return self.firstName + ' ' + self.lastName;
    }

    this.addressMe = function(msg) {
        return fullName() + ": " + msg;
    };
};

那么你可以这样做：

var bob = new People("Bob", "Scum");
alert( bob.addressMe("stand and deliver") ); // "Bob Scum: stand and deliver"

Answer 2

Pointy 对私有 shared 成员使用了错误的模式，该模式适用于私有 instance specific 成员，并且每个实例都有自己的 addressMe 函数。

私有共享成员的模式可以将原型包装在 IIFE 中：

var People = function(firstName, lastName){
    // public
    this.firstName = firstName;
    this.lastName  = lastName;
};

(function(){
  // private shared member
  var full_name = function(){
      return this.firstName + ' ' + this.lastName;
  };
  //privileged member that has access to private shared
  //  fullName is a closure and full_name is available
  //  in the colosure scope created for this function
  People.prototype.fullName = function(){
    //calling private member, make sure you set
    //  the invoking object (this) by using
    //  Function.prototype.call
    console.log(full_name.call(this));
    return this;
  };
}());

var person = new People('John', 'Smith');
person.fullName();

您不能将此模式用于特定于实例的私有成员，但函数 full_name 不是特定于实例的。

可以在here 找到受保护的模式以及有关原型和构造函数的更多信息。