javascript 中没有关键字 var 的范围 [英] scope in javascript without the keyword var

问题描述

function jjj(asi) {
  asi=3;
}

jjj();
console.log(asi);

这里我认为 asi 是一个全局变量，但是在运行这段代码时，它给出了 asi 没有定义.

Here I am thinking that asi is a global variable but while running this code it is giving that asi is not defined.

根据书籍和官方文档，我研究过，如果你提到变量名而没有关键字 var 那么它就变成全局的，所以我认为同样的规则也适用于 asi 变量

As per the books and official doc I have studied that if you mention the variable name without the keyword var then it becomes global so I think same rule applies to asi variable also

推荐答案

这里我认为 asi 是一个全局变量，但是在运行此代码时，它给出了 asi 未定义

here I am thinking that asi is a global variable but while running this code it is giving that asi is not defined

它会创建一个隐式全局如果你没有将它声明为一个参数，例如:

It would be creating an implicit global if you weren't declaring it as a parameter instead, e.g.:

function jjj() {
//           ^---------- removed `asi` here
  asi = 3;
}
jjj();
console.log(asi);

请注意，隐式全局变量一个非常糟糕的主意(我称我的博客文章隐式全局变量的恐怖是有原因的)，您应该使用严格模式 使他们成为他们一直应该出现的错误:

Note that implicit globals are a really bad idea (I called my blog post on them The Horror of Implicit Globals for a reason) and you should use strict mode to make them the errors they always should have been:

"use strict";
function jjj() {
  asi = 3; // ReferenceError: asi is not defined
}
jjj();
console.log(asi);

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