javascript - 拦截 javascript 对象的属性/方法的获取

Question

如何/我可以拦截请求以获取 javascript 对象的方法/属性并返回自定义值。

例如，

var obj = {
    // This "get" method is where I want to replace the call to get a property of an object.
    get : function(propertyName) {
        if (propertyName == 'somePropertyThatDoesntExist') { return 1; }
        else { return 0; } // Something like this?
};

// Either this method of access,
var myValue1 = obj.somePropertyThatDoesntExist
var myValue2 = obj.someOtherPropertyThatDoesntExist

// Alternatively, 
var myValue3 = obj['somePropertyThatDoesntExist']
var myValue4 = obj['someOtherPropertyThatDoesntExist']

所以 myValue1 和 myValue3 的值为 1，而 myValue2 和 myValue4 的值为 0。

目前， myValue1, 2, 3, 4 都将是“未定义的”。

Answer 1

我认为没有办法拦截对未定义属性的访问。您可以随时执行以下操作

if(obj.someProperty != undefined)
    obj.someProperty // access the property

或者，您可以编写一个 get 方法

var obj = {
    someProperty : "val",
    get : function(propertyName) {

        if (this[propertyName] == undefined) { return 0; }
        else { return this[propertyName]; } 
    }
};

并像使用它一样

obj.get("someProperty") // returns 0 if property is not defined. 

Answer 2

这在 JavaScript ES6 中使用Proxy对象是可能的

var obj = {}

// Wrap the object using Proxy
obj = new Proxy(obj, {
  get : function (target, propertyName) {
    if (propertyName === 'somePropertyThatDoesntExist') { return 1; }
    else { return 0; }
  }
})

var myValue1 = obj.somePropertyThatDoesntExist       // 1
var myValue2 = obj.someOtherPropertyThatDoesntExist  // 0