Javascript本地和全局变量混淆 [英] Javascript local and global variable confusion

问题描述

我是JavaScript的新手，我在本地和全局变量范围上做了一些练习，以下是我的代码（小提琴）：

I am new to JavaScript and I was doing some practices on local and global variable scopes, following is my code(fiddle):

var myname = "initial"
function c(){
    alert(myname);
    var myname = "changed";
    alert(myname);
}
c();

当第一个调用alert，它显示 myname 为undefined。所以我的困惑是为什么我无法访问 myname 的全局实例，如果我没有定义 myname 在函数中，它将正常工作。

when the first alert is called, it is showing myname as undefined. so my confusion is why I am not able to access a global instance of myname and if I don't define myname within the function then it will work fine.

推荐答案

在Javascript中，变量声明会自动移动到函数的顶部。因此，解释器会使它看起来更像这样：

In Javascript, the variable declarations are automatically moved to the top of the function. So, the interpreter would make it look more like this:

var myname = "initial"
function c(){
    var myname;
    // alerts undefined
    alert(myname);
    myname = "changed";
    // alerts changed
    alert(myname);
}
c();

这称为'吊装'。

由于吊装以及任何变量的范围是其声明的函数这一事实，标准做法是列出所有变量函数的顶部以避免这种混淆。

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.

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