在同一范围内声明两次Javascript变量 - 这是一个问题吗？ [英] Declaring a Javascript variable twice in same scope - Is it an issue?

问题描述

以下代码是否会导致任何问题？：

Would the following code cause any issues?:

var a = 1;
var a = 2;

我的理解是javascript变量是在范围的开头声明的。例如：

My understanding is that javascript variables are declared at the start of the scope. For example:

var foo = 'a';
foo = 'b';
var bar = 'c';

处理为：

var foo;
var bar;
foo = 'a';
foo = 'b';
bar = 'c';

因此，我的初始代码段将成为：

Therefore would my initial code snippet become:

var a;
a = 1;
a = 2;

或者它会变成：

var a;
var a;
a = 1;
a = 2;

我知道在同一范围内两次声明一个javascript变量不是好习惯，但我'我对这样做的影响更感兴趣。

I understand that declaring a javascript variable twice in the same scope isn't good practice, but I'm more interested in the impact of doing this.

推荐答案

正如你所说，两次以上的同一个var，JavaScript会移动声明到范围的顶部然后你的代码会变成这样：

As you said, by twice of more the same var, JavaScript moves that declaration to the top of the scope and then your code would become like this:

var a;
a = 1;
a = 2;

因此，它不会给我们任何错误。

Therefore, it doesn't give us any error.

循环的同样的行为（它们的标题中没有本地范围），所以下面的代码非常常见：

This same behaviour occurs to for loops (they doesn't have a local scope in their headers), so the code below is really common:

for (var i = 0; i < n; i++) {
    // ...
}
for (var i = 0; i < m; i++) {
    // ...
}

这就是为什么像 Douglas Crockford 这样的JavaScript大师建议程序员手动将这些声明移到顶端范围：

That's why JavaScript gurus like Douglas Crockford suggest programmers to manually move those declarations to the top of the scope:

var i;    // declare here
for (i = 0; i < n; i++) {    // initialize here...
    // ...
}
for (i = 0; i < m; i++) {    // ... and here
    // ...
}

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