delete关键字在全局变量上的不同行为 [英] Different behaviour of delete keyword on global variables

问题描述

请考虑以下代码段（小提琴此处）：

Please consider the following snippet (fiddle here):

​var a;

​a = 1;
console.log(delete a​); // prints 'false'

​b = 1;
console.log(delete b);​ // prints 'true'​​​​

为什么 delete 关键字对全局变量的行为不同 a 和 b ？

Why does the delete keywords behave differently on the global variables a and b?

推荐答案

来自MDN文档：

删除运算符从对象中删除属性。

The delete operator removes a property from an object.

全局变量（不含 var ）是全局对象的属性（通常是窗口），因此可以删除。

A global variable (without var) is a property on the global object (typically window), so can be deleted.

A var 不是全局变量，而是外部作用域中的局部变量 - 不是全局对象的属性 - 所以 delete 不会删除它。来自这些文档：

A var is not a global variable, but a local variable in the outer scope - not a property of the global object - so delete does not delete it. From those docs:

x = 42;     // creates the property x on the global object
var y = 43; // declares a new variable, y

delete x;   // returns true  (x is a property of the global object and can be deleted)
delete y;   // returns false (delete doesn't affect variable names)

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