变量 === 未定义 vs. typeof 变量 === “未定义" [英] variable === undefined vs. typeof variable === "undefined"

问题描述

jQuery Core Style Guidelines 建议了两种不同的检查方式是否定义了变量.

The jQuery Core Style Guidelines suggest two different ways to check whether a variable is defined.

• 全局变量:typeof variable ===未定义"
• 局部变量:variable === undefined
• 属性:object.prop === undefined

为什么 jQuery 对全局变量使用一种方法而对局部变量和属性使用另一种方法?

Why does jQuery use one approach for global variables and another for locals and properties?

推荐答案

对于未声明的变量，typeof foo 将返回字符串字面量 "undefined"，而身份检查foo === undefined 会触发错误foo is not defined".

For undeclared variables, typeof foo will return the string literal "undefined", whereas the identity check foo === undefined would trigger the error "foo is not defined".

对于局部变量(您知道已在某处声明)，不会发生此类错误，因此需要进行身份检查.

For local variables (which you know are declared somewhere), no such error would occur, hence the identity check.

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