JavaScript Accessor实现说明

第一种算是比较常见了,通过闭包Store Value从而实现accessor,适用于所有浏览器.

function Sandy(val){ 
var value = val; 
this.getValue = function(){ 
return value; 
}; 
this.setValue = function(val){ 
value = val; 
}; 
} 
//usage 
var sandy = new Sandy("test"); 
sandy.value 
// => undefined 
sandy.setValue("test2") 
sandy.getValue

下面是JavaScript权威指南（中文第五版）中P152页使用闭包的一个例子.

function makeProperty(o, name, predicate) { 
var value; //This is property value; //The setter method simply returns the value 
o['get' + name] = function() { return value;}; 
//The getter method stores the value or throws an exception if 
//the predicate rejects the value 
o['set' + name] = function(v) { 
if (predicate && !predicate(v) { 
throw 'set' + name + ': invalid value ' + v; 
} else { 
value = y; 
} 
} 
} 
//The following code demenstrates the makeProperty() method 
var o = {}; // Here is an empty object 
//Add property accessor methods getName and setName 
//Ensure that only string values are allowed 
makeProperty(o, 'Name', function(x) { return typeof x == 'string'; }); 
o.setName('Frank'); 
//Set the property value; 
print(o.getName()); 
//Get the property value 
o.setName(0); //Try to set a value of the wrong type

第二种方法是使用__defineSetter__与__defineGetter__来实现accessor,看下划线就知道它们并非标准,适用于Firefox 2.0+, Safari 3.0+, Google Chrome 1.0+ 和 Opera 9.5+ ,方法使用见MDN.

function Sandy(val){ 
var value = val, 
_watch = function(newVal) { 
console.log('val is Changed to : ' + newVal); 
} this.__defineGetter__("value", function(){ 
return value; 
}); 
this.__defineSetter__("value", function(val){ 
value = val; 
_watch(val); 
}); 
} 
var sandy = new Sandy("test"); 
sandy.value 
// => test 
sandy.value = "test2"; 
// => 'val is Changed to : test2' 
sandy.value 
// => "test2"

　除了__defineG/Setter__外, 你还可以使用'set'、'get'关键字在在原型对象上定义accessor,对于单个对象同样适用, 适用于Firefox 2.0+, Safari 3.0+, Google Chrome 1.0+ 和 Opera 9.5+.

function Sandy(val){ 
this.value = val; 
} Sandy.prototype = { 
get value(){ 
return this._value; 
}, 
set value(val){ 
this._value = val; 
} 
}; 
//Or 
var sandy = { 
'_value' : 'sandy', 
get value() { 

return this._value; 
}, 
set value(val) { 

this._value = val; 
} 
}

最后一种方法,用到了Object的静态方法defineProperty,作用于单个对象,该方法应该属于ES5的范畴了,目前似乎只有Chrome 支持这种方法，其实Ie8也支持,但操作对象仅限于Dom节点(Dom node),见IEBlog,该方法的使用见MDN.

var sandy = {}, rValue; 
Object.defineProperty(sandy, 'value' , 
{ 
'set' : function(val) { 
rValue = val; 
}, 
'get' : function() { 
return rValue; 
}, 
'enumerable' : true, 
'configurable' : true 
} 
) 
//Ie8+ 
Object.defineProperty(document.body, "description", { 
get : function () { 
return this.desc; 
}, 
set : function (val) { 
this.desc = val; 
} 
}); 
document.body.description = "Content container"; 
// document.body.description will now return "Content container"

‘enumerable','configuralbe' 属于ES5规范中的Property Attributes(属性特性),在这里就不做讨论了,有兴趣的Google或者直接去看ES5的文档. ^ ^