为什么这些JavaScript函数未添加到该对象？

Question

我想为基于Json文件的对象编写getter和setter函数。 我复制了约翰·雷西格（John Resig）的书的以下代码（Appress Pro JavaScript Techniques），但是它不起作用，并且这些函数没有添加到对象中。您能告诉我为什么，什么是正确的代码？

// 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

您已经使用一个函数创建了一个闭包，但是您忘记了将i传递给它。 您还需要在函数内部this进行不同的引用，因为上下文将其更改为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

在您的IIFE中， this实际上是window 。 您必须使用Function.prototype.call来指定上下文：

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);
    }
}

或保留另一个变量的引用：

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);
    }
}

如果处于严格模式，则代码将引发错误，而不是误导成功：

TypeError: Cannot set property 'getname' of undefined