Understanding the Distinction: variable === undefined vs. typeof variable === "undefined"

In the jQuery Core Style Guidelines, two methods are proposed to verify if a variable is defined:

• Global Variables: typeof variable === "undefined"
• Local Variables and Properties: variable === undefined

Why this difference?

The explanation lies in the behavior of these operators when dealing with undeclared variables.对于未申报的变量，FOO TypeOf Type将返回“未定义”作为字符串。但是，身份检查foo ===未定义会引起错误“未定义foo”。

因此，jQuery使用typeof Operator进行全局变量（可能被声明或可能不会声明），以避免潜在的错误。对于确保声明的本地变量和属性，由于其简单性，因此优选身份检查。