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"
六、内置属性和DontDelete
一句话：属性中一个独特的特性(DontDelete)控制着这个属性是否能被删除。注意，对象的内置属性（即对象的预定义属性）有DontDelete 特性，因此不能被删除。特定的Arguments 变量（或者，正如我们现在了解的，激活对象的属性），任何函数实例的length属性也拥有DontDelete 特性。
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"
})();
与函数参数相对应的创建的属性也有DontDelete 特性，因此也不能被删除。
复制代码 代码如下:
(function(foo, bar){
delete foo; // false
foo; // 1
delete bar; // false
bar; // 'blah'
})(1, 'blah');
七、未声明的赋值
简单地就是未声明的赋值在一个全局对象上创建一个可删除的属性。
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;//可理解为 window.bar=2; 根据上面的第五点是可以删除的
delete foo; // false
typeof foo; // "number"
delete bar; // true
typeof bar; // "undefined"
请注意，DontDelete特性是在属性创建的过程中确定的，后来的赋值不会修改现有属性已经存在的特性，理解这一点很重要。
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"
八、Eval code
在Eval中创建的变量或方法比较特别，没有DontDelete特性，也就是说可以删除。
复制代码 代码如下:
eval("var x = 1;");
console.log(x); // 1
delete x;
console.log(typeof x); // undefined
eval("function test(){ var x=1; console.log(delete x);/* false */;return 1;}");
console.log(test()); // 1
delete test;
console.log(typeof test); // undefined
注意，这里说的在Eval中创建的变量或方法不包括方法内部的变量或方法，如上面代码中的红色部分，仍然跟之前讲的一致：不能被删除。
九、FireBug的困惑
我们看一段在FireBug中执行的代码结果：
复制代码 代码如下: