本页为Object.defineProperty()
提供一个附加示例。
使用二进制标志代替属性来描述对象
如果你通过二进制标志
使用相同的描述对象,一次次重新定义每个属性。
var oDesc = {}; function setProp (nMask, oObj, sKey, vVal_fGet, fSet) { if (nMask & 8) { // accessor descriptor if (vVal_fGet) { oDesc.get = vVal_fGet; } else { delete oDesc.get; } if (fSet) { oDesc.set = fSet; } else { delete oDesc.set; } delete oDesc.value; delete oDesc.writable; } else { // data descriptor if (arguments.length > 3) { oDesc.value = vVal_fGet; } else { delete oDesc.value; } oDesc.writable = Boolean(nMask & 4); delete oDesc.get; delete oDesc.set; } oDesc.enumerable = Boolean(nMask & 1); oDesc.configurable = Boolean(nMask & 2); Object.defineProperty(oObj, sKey, oDesc); return oObj; } /* * :: function setProp :: * * nMask is a bitmask: * flag 0x1: property is enumerable, * flag 0x2: property is configurable, * flag 0x4: property is writable, * flag 0x8: property is accessor descriptor. * oObj is the object on which to define the property; * sKey is the name of the property to be defined or modified; * vVal_fGet is the value to assign to a data descriptor or the getter function * to assign to an accessor descriptor (depending on the bitmask); * fSet is the setter function to assign to an accessor descriptor; * * Bitmask possible values: * * 0 : readonly data descriptor - not configurable, not enumerable (0000). * 1 : readonly data descriptor - not configurable, enumerable (0001). * 2 : readonly data descriptor - configurable, not enumerable (0010). * 3 : readonly data descriptor - configurable, enumerable (0011). * 4 : writable data descriptor - not configurable, not enumerable (0100). * 5 : writable data descriptor - not configurable, enumerable (0101). * 6 : writable data descriptor - configurable, not enumerable (0110). * 7 : writable data descriptor - configurable, enumerable (0111). * 8 : accessor descriptor - not configurable, not enumerable (1000). * 9 : accessor descriptor - not configurable, enumerable (1001). * 10 : accessor descriptor - configurable, not enumerable (1010). * 11 : accessor descriptor - configurable, enumerable (1011). * * Note: If the flag 0x8 is setted to "accessor descriptor" the flag 0x4 (writable) * will be ignored. If not, the fSet argument will be ignored. */ // creating a new empty object var myObj = {}; // adding a writable data descriptor - not configurable, not enumerable setProp(4, myObj, 'myNumber', 25); // adding a readonly data descriptor - not configurable, enumerable setProp(1, myObj, 'myString', 'Hello world!'); // adding an accessor descriptor - not configurable, enumerable setProp(9, myObj, 'myArray', function() { for (var iBit = 0, iFlag = 1, aBoolArr = [false]; iFlag < this.myNumber + 1 || (this.myNumber & iFlag); iFlag = iFlag << 1 ) { aBoolArr[iBit++] = Boolean(this.myNumber & iFlag); } return aBoolArr; }, function(aNewMask) { for (var nNew = 0, iBit = 0; iBit < aNewMask.length; iBit++) { nNew |= Boolean(aNewMask[iBit]) << iBit; } this.myNumber = nNew; }); // adding a writable data descriptor (undefined value) - configurable, enumerable setProp(7, myObj, 'myUndefined'); // adding an accessor descriptor (only getter) - configurable, enumerable setProp(11, myObj, 'myDate', function() { return new Date(); }); // adding an accessor descriptor (only setter) - not configurable, not enumerable setProp(8, myObj, 'myAlert', null, function(sTxt) { alert(sTxt); }); myObj.myAlert = myObj.myDate.toLocaleString() + '\n\n' + myObj.myString + '\nThe number ' + myObj.myNumber + ' represents the following bitmask: ' + myObj.myArray.join(', ') + '.'; // listing the enumerable properties var sList = 'Here are the enumerable properties of myObj object:\n'; for (var sProp in myObj) { sList += '\nmyObj.' + sProp + ' => ' + myObj[sProp] + ';' } alert(sList);
创建一个新的Object.setProperty()
方法
你可以对通过匿名构造函数和Object
的名为setProperty()
的自定义方法获取的描述符对象做同样的事情:
// creating a new Object method named Object.setProperty() new (function() { var oDesc = this; Object.setProperty = function(nMask, oObj, sKey, vVal_fGet, fSet) { if (nMask & 8) { // accessor descriptor if (vVal_fGet) { oDesc.get = vVal_fGet; } else { delete oDesc.get; } if (fSet) { oDesc.set = fSet; } else { delete oDesc.set; } delete oDesc.value; delete oDesc.writable; } else { // data descriptor if (arguments.length > 3) { oDesc.value = vVal_fGet; } else { delete oDesc.value; } oDesc.writable = Boolean(nMask & 4); delete oDesc.get; delete oDesc.set; } oDesc.enumerable = Boolean(nMask & 1); oDesc.configurable = Boolean(nMask & 2); Object.defineProperty(oObj, sKey, oDesc); return oObj; }; })(); // creating a new empty object var myObj = {}; // adding a writable data descriptor - not configurable, not enumerable Object.setProperty(4, myObj, 'myNumber', 25); // adding a readonly data descriptor - not configurable, enumerable Object.setProperty(1, myObj, 'myString', 'Hello world!'); // etc. etc.
注意:
Object.setProperty()
方法是一个 JavaScript 新的原生方法的提议(参见
ECMAScript bug 335)。
语法
Object.setProperty(bitmask, obj, prop[, value/getter[, setter]])
参数
-
bitmask
- 描述符位掩码(见下文)。
-
obj
- 目标对象。
-
prop
- 将要被定义或修改的属性。
-
value/getter
- 可选。要分配给数据描述符的值或要分配给访问器描述符的getter函数(取决于位掩码)。
-
setter
-
可选。setter 函数将分配到可访问描述符。如果
0x8
标识已经设置, 这个参数将会被忽略。
描述
除了用描述符位掩码替换的描述符对象外,非原生Object.setProperty()
方法与原生Object.defineProperty()
方法类似。 bitmask
参数具有以下结构:
-
flag
0x1
- 属性可枚举。
-
flag
0x2
- 属性可配置。
-
flag
0x4
- 属性可写入。
-
flag
0x8
- 属性为可访问描述符(其实就是一个 key: function)
所以,描述位掩码可以有以下可能的值:
0
: 位掩码表示只读数据描述符 — not configurable, not enumerable (0000
).1
: 位掩码表示只读数据描述符 — not configurable, enumerable (0001
).2
: 位掩码表示只读数据描述符 — configurable, not enumerable (0010
).3
: 位掩码表示只读数据描述符 — configurable, enumerable (0011
).4
: 位掩码表示可写数据描述符 — not configurable, not enumerable (0100
).5
: 位掩码表示可写数据描述符 — not configurable, enumerable (0101
).6
: T位掩码表示可写数据描述符 — configurable, not enumerable (0110
).7
: T位掩码表示可写数据描述符 — configurable, enumerable (0111
).8
: 位掩码表示可访问数据描述符 — not configurable, not enumerable (1000
).9
: 位掩码表示可访问数据描述符 — not configurable, enumerable (1001
).10
: 位掩码表示可访问数据描述符 — configurable, not enumerable (1010
).11
: 位掩码表示可访问数据描述符 — configurable, enumerable (1011
).
注意:如果标志
0x8
被设置为访问描述符,则标志
0x4
(可写)将被忽略。否则,
setter
参数将被忽略。
HTMLSelectElement.selectedIndex 实现
你也可以使用本地方法 Object.defineProperty()
。用以下例子展现如何实现HTMLSelectElement
和 selectedIndex
单选框组属性。
<!doctype html> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> <title>Radio group selectedIndex example</title> <script type="text/javascript"> Object.defineProperty(NodeList.prototype, 'selectedIndex', { get: function() { var nIndex = this.length - 1; while (nIndex > -1 && !this[nIndex].checked) { nIndex--; } return nIndex; }, set: function(nNewIndex) { if (isNaN(nNewIndex)) { return; } var nOldIndex = this.selectedIndex; if (nOldIndex > -1) { this[nOldIndex].checked = false; } if (nNewIndex > -1) { this[nNewIndex].checked = true; } }, enumerable: true, configurable: false }); // try it! function checkForm() { var nSelectedIndex = document.myForm.myRadioGroup.selectedIndex; if (nSelectedIndex < 0) { alert('Select a gadget!!'); return false; } alert('Congratulations!! You selected the ' + document.myForm.myRadioGroup[nSelectedIndex].value + '.'); return true; } </script> </head> <body> <form name="myForm" onsubmit="return(checkForm());"> <fieldset><legend>Select a gadget</legend> <p><input type="radio" name="myRadioGroup" id="ourShirt" value="shirt" /> <label for="ourShirt">shirt</label><br /> <input type="radio" name="myRadioGroup" id="ourPants" value="pants" /> <label for="ourPants">pants</label><br /> <input type="radio" name="myRadioGroup" id="ourBelt" value="belt" /> <label for="ourBelt">belt</label><br /> <input type="radio" name="myRadioGroup" id="ourShoes" value="shoes" /> <label for="ourShoes">shoes</label></p> <p><span style="cursor:pointer;text-decoration:underline;color:#0000ff;" onclick="document.myForm.myRadioGroup.selectedIndex=2;">Select our favorite gadget ;-)</span></p> <p><input type="submit" value="Order!" /> </fieldset> </form> </body> </html>