Javascript 局部变量和全局变量混淆
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Javascript local and global variable confusion
提问by Bharat Soni
I am new to JavaScript and I was doing some practices on local and global variable scopes, following is my code(fiddle):
我是 JavaScript 的新手，我正在对本地和全局变量范围做一些练习，以下是我的代码（小提琴）：
var myname = "initial"
function c(){
alert(myname);
var myname = "changed";
alert(myname);
}
c();when the first alert is called, it is showing mynameas undefined. so my confusion is why I am not able to access a global instance of mynameand if I don't define mynamewithin the function then it will work fine.
当第一个警报被调用时，它显示myname为未定义。所以我的困惑是为什么我无法访问的全局实例，myname如果我不在myname函数中定义，那么它会正常工作。
采纳答案by EmptyArsenal
In Javascript, the variable declarations are automatically moved to the top of the function. So, the interpreter would make it look more like this:
在 Javascript 中，变量声明会自动移动到函数的顶部。所以，解释器会让它看起来更像这样：
var myname = "initial"
function c(){
var myname;
// alerts undefined
alert(myname);
myname = "changed";
// alerts changed
alert(myname);
}
c();This is called 'hoisting'.
这称为“提升”。
Due to hoisting and the fact that the scope for any variable is the function it's declared in, it's standard practice to list all variables at the top of a function to avoid this confusion.
由于提升以及任何变量的作用域都是它在其中声明的函数这一事实，标准做法是在函数顶部列出所有变量以避免这种混淆。
回答by Sridhar R
It is not replace the global variable. What is happening is called "variable hoisting". That is, var myname;gets inserted at the top of the function. Always initialize your variables before you use them
- try this:
它不是替换全局变量。正在发生的事情称为“变量提升”。也就是说，var myname;被插入到函数的顶部。在使用变量之前始终初始化它们 - 试试这个：
var myname = "initial";
function c() {
alert(myname);
myname = "changed";
alert(myname);
}
c();