var GLOBAL_OBJECT = this;
/* `foo` is a property of a Global object.
It is created via variable declaration and so has DontDelete attribute.
This is why it can not be deleted. */
var foo = 1;
delete foo; // false
typeof foo; // "number"
/* `bar` is a property of a Global object.
It is created via function declaration and so has DontDelete attribute.
This is why it can not be deleted either. */
function bar(){}
delete bar; // false
typeof bar; // "function"
/* `baz` is also a property of a Global object.
However, it is created via property assignment and so has no DontDelete attribute.
This is why it can be deleted. */
GLOBAL_OBJECT.baz = 'blah';
delete GLOBAL_OBJECT.baz; // true
typeof GLOBAL_OBJECT.baz; // "undefined"
内置对象和DontDelete
所以, 这就是有关它(DontDelete)的所有: 属性的一个特殊特性, 用来控制这个属性是否能够被删除. 注意, 有些内置对象的属性是指定含有DontDelete的, 所以无法被删除. 如特殊的arguments变量(或者, 正如我们现在所知道的, 一个活动对象的属性)拥有DontDelete. 函数实例的length属性也具有DontDelete属性.
复制代码 代码如下:
(function(){
/* can't delete `arguments`, since it has DontDelete */
delete arguments; // false
typeof arguments; // "object"
/* can't delete function's `length`; it also has DontDelete */
function f(){}
delete f.length; // false
typeof f.length; // "number"
})();
函数参数所对应的属性也是从建立开始就拥有DontDelete特性的, 所以我们也无法删除它.
复制代码 代码如下:
(function(foo, bar){
delete foo; // false
foo; // 1
delete bar; // false
bar; // 'blah'
})(1, 'blah');
未声明的赋值:
你可能还记着, 未声明的赋值会在全局对象上建立一个属性, 除非这个属性已经在这个作用域链中全局对象之前的其它地方被找到. 并且, 现在我们知道属性赋值和变量声明的不同之处——后者会设置DontDelete属性, 但前者不会. 我们必须清楚, 为什么未声明的赋值会建立一个可删除的属性.
复制代码 代码如下:
var GLOBAL_OBJECT = this;
/* create global property via variable declaration; property has DontDelete */
var foo = 1;
/* create global property via undeclared assignment; property has no DontDelete */
bar = 2;
delete foo; // false
typeof foo; // "number"
delete bar; // true
typeof bar; // "undefined"
请注意: 特性是在属性被创建时被决定的, 之后的赋值不会修改已存在属性的特性. 理解这一点区别非常重要.
复制代码 代码如下:
/* `foo` is created as a property with DontDelete */
function foo(){}
/* Later assignments do not modify attributes. DontDelete is still there! */
foo = 1;
delete foo; // false
typeof foo; // "number"
/* But assigning to a property that doesn't exist,
creates that property with empty attributes (and so without DontDelete) */
this.bar = 1;
delete bar; // true
typeof bar; // "undefined"
Firebug的困惑:
在Firebug中发生了什么? 为什么在console中声明的变量可以被删除, 这不是违背了我们之前所学到的知识么? 嗯, 就像我之前所说的那样, Eval代码在面对变量声明时会有特殊的表现. 在Eval中声明的变量实际上是作为不带DontDelete特性的属性被创建的.
复制代码 代码如下:
eval('var foo = 1;');
foo; // 1
delete foo; // true
typeof foo; // "undefined"
同样, 类似的, 当在函数代码中调用时:
复制代码 代码如下:
(function(){
eval('var foo = 1;');
foo; // 1
delete foo; // true
typeof foo; // "undefined"
})();
这就是Firebug反常行为的依据. 在console中的所有文本都会被当做Eval代码来解析和执行, 而不是全局或函数代码. 显然, 这里声明的所有变量最后都会成为不带DontDelete特性的属性, 所以它们都能被轻松删除. 我们需要了解这个在全局代码和Firebug控制台之间的差异.
通过Eval来删除变量:
这个有趣的eval行为, 再加上ECMAScript的另一个方面, 可以在技术上允许我们删除"non-deletable"的属性. 有关函数声明的一点是, 它们能够覆盖相同执行上下文中同名的变量.
复制代码 代码如下:
function x(){ }
var x;
typeof x; // "function"