函数申明和变量申明的微妙区别

<script>
var test1 = 1;
function test1(){};//函数申明不能覆盖变量申明？
alert(test1);
</script>
<script>
var test2 = 1;
eval('function test2(){};');
alert(test2);//函数申明怎么又覆盖了变量申明？
</script>

评论

8 楼 jindw 2007-05-09  

没找到合适的解释

7 楼 jindw 2007-05-09  

12.2 Variable statement Description
If the variable statement occurs inside a FunctionDeclaration, the variables are defined with function-local
scope in that function, as described in s10.1.3. Otherwise, they are defined with global scope (that is, they
are created as members of the global object, as described in 10.1.3) using property attributes { DontDelete
}. Variables are created when the execution scope is entered. A Block does not define a new execution
scope. Only Program and FunctionDeclaration produce a new scope. Variables are initialised to undefined
when created. A variable with an Initialiser is assigned the value of its AssignmentExpression when the
VariableStatement is executed, not when the variable is created.


函数申明：
Semantics
The production FunctionDeclaration : function Identifier ( FormalParameterListopt ) { FunctionBody }
is processed for function declarations as follows:
1. Create a new Function object as specified in 13.2 with parameters specified by FormalParameterList, and
body specified by FunctionBody. Pass in the scope chain of the running execution context as the Scope.
2. Create a property of the current variable object (as specified in 10.1.3) with name Identifier and value
Result(1).

6 楼 jindw 2007-05-09  

2007-05-09 18:49 
2007-05-09 18:52 
又一次把java eye当QQ用了

to dlee:
这些东西在ECMA262－3里面有描述吗？
我初看一下，没找到呢。

5 楼 dlee 2007-05-09  

是的，这就是上面我最后补充的一句话的情况：

引用

当然，如果在外部的代码中包含有函数调用，其调用的这个函数是会在调用之前创建的。

4 楼 jindw 2007-05-09  

呵呵，还有更奇怪的。

<script> 
alert(test1());//这样也行？？？？ 
var test1 = 1; 
function test1(){return 'xxx'};
</script> 

3 楼 dlee 2007-05-09  

不是这个意思，出现这个情况的原因是这样：对于页面中的脚本，JavaScript引擎一般是先执行完所有外部的代码之后，才会创建声明的函数的。
所以jindw上面的第一段代码，其实相当于是：

<script>
var test1 = 1;
alert(test1);
function test1(){};//函数申明不能覆盖变量申明？
</script>

而第二段代码呢，

<script>
var test2 = 1;
eval('function test2(){};');
alert(test2);//函数申明怎么又覆盖了变量申明？
</script>

因为eval这条语句属于外部代码，所以执行的顺序与jindw写的顺序是相同的。

当然，如果在外部的代码中包含有函数调用，其调用的这个函数是会在调用之前创建的。

2 楼 sp42 2007-05-09  

var test2 = 1;
eval('test2=new Function();')
alert(test2);
如果这样就可以覆盖掉了（指向一个匿名函数）

1 楼 sp42 2007-05-09  

估计是eval()内部apply()了一下，把这个function挂到window上了