容易被忽略的JS脚本特性
2024-05-06 14:23:42
供稿：网友
一、容易被忽略的局部变量
代码如下:
var a = 5;
(function(){
alert(a);
var a = a ++;
alert(a);
})()
alert(a);
思考这段代码的执行结果。
执行后，看看是否和你想象的一致？
ok，这段代码里核心的知识点是 var a = a++，其中两个变量 a 都是匿名函数内部的局部变量，是同一个，和全局变量 a 是不一样的。
为什么？我们来看看ECMA规范对变量声明语句的定义：
代码如下:
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.
声明中提到：进入作用域环境后，变量就会被创建，并赋予初始值undefined。在变量声明语句执行时才会把赋值表达式的值指派给该变量，而并不是在该变量被创建时。
因此上面的代码可以等价于:
代码如下:
var a;
a = 5;
(function(){
var a;
alert(a);
a = a ++;
alert(a);
})()
alert(a);
这样应该会更容易理解了。
二、容易被忽略的全局变量
代码如下:
(function(){
var a = b = 5;
})()
alert(b);
这是玉伯几天前分享到的知识点，蛮有意义的，在此也做个分析。
首先，考虑执行结果为什么是：5。
ok,原因出在 var a = b = 5 这句。
为深入分析这个语句，我们继续要参照ECMA规范对声明语句的定义：
var a = b = 5;等同为 var a; a = b = 5;两条语句，后者是赋值表达式，其在ECMA中的定义是这样的：
代码如下:
Simple Assignment ( = )
The production AssignmentExpression : LeftHandSideExpression =
AssignmentExpression is evaluated as follows:
1. Evaluate LeftHandSideExpression.
2. Evaluate AssignmentExpression.
3. Call GetValue(Result(2)).
4. Call PutValue(Result(1), Result(3)).
5. Return Result(3).
对于a = b = 5;先执行左边表达式 a，这是一个标识符表达式，根据规范第 10.1.4，其执行方式如下：
代码如下:
During execution, the syntactic production PrimaryExpression : Identifier
is evaluated using the following algorithm:
1. Get the next object in the scope chain. If there isn't one, go to step 5.
2. Call the [[HasProperty]] method of Result(1), passing the Identifier as
the property.
3. If Result(2) is true, return a value of type Reference whose base
