使用var和非var变量删除运算符 [英] delete operator with var and non var variables

问题描述

我尝试使用delete运算符在javascript中删除变量，但发现了一些问题。
你们可以解释下面的代码，以及它为什么会发生

I tried delete a variable in javascript using delete operator, but found some issue. Can you guys please explain the below code, and why it is happening

>> var a = 5;

>> delete a
false

>>a
5

>> b=5;

>>delete b
true

>>b
ReferenceError b is not defined

为什么var a = 5和b = 5不同？

why var a = 5 and b = 5 are different?

推荐答案

当使用变量声明（即使用 var ）创建变量时，会创建变量并将其deleteable标志设置为 false 。

When a variable is created using a variable declaration (i.e. using var) then it is created with its deleteable flag set to false.

当变量由赋值隐式创建而未声明时，其可删除标志设置为 true 。

When a variable is created implicitly by assignment without being declared, its deleteable flag is set to true.

全局执行上下文的一个特点是变量也是全局对象的属性（这不会​​发生在函数或eval代码中）。所以当你这样做时：

It is a peculiarity of the global execution context that variables are also made properties of the global object (this doesn't happen in function or eval code). So when you do:

var a;

然后 a 是变量，也是全局的属性（窗口）在浏览器中）对象并将其可删除标志设置为 false 。但是：

Then a is a variable and also a property of the global (window in a browser) object and has its deleteable flag set to false. But:

a = 'foo';

将 a 创建为没有声明的全局变量，因此其可删除标志设置为 true 。

creates a as a global variable without a declaration, so its deleteable flag is set to true.

结果是您可以删除隐式创建的全局变量，但不能删除由声明创建的变量（包括函数声明） 。

The result is that you can delete global variables created implicitly, but not those created by declarations (including function declarations).

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