变量=== undefined vs. typeof variable ===" undefined" [英] variable === undefined vs. typeof variable === "undefined"
问题描述
jQuery核心风格指南提供了两种不同的方法来检查是否定义了变量。
- 全局变量:
typeof variable ===undefined
- 本地变量:
变量=== undefined
- 属性:
object.prop = == undefined
为什么jQuery对全局变量使用一种方法而对本地和属性使用另一种方法?对于未声明的变量, typeof foo
将返回字符串文字undefined
,而身份检查 foo === undefined
会触发错误foo未定义 。
对于局部变量(您知道在某处声明),不会发生此类错误,因此进行身份检查。
The jQuery Core Style Guidelines suggest two different ways to check whether a variable is defined.
- Global Variables:
typeof variable === "undefined"
- Local Variables:
variable === undefined
- Properties:
object.prop === undefined
Why does jQuery use one approach for global variables and another for locals and properties?
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".
For local variables (which you know are declared somewhere), no such error would occur, hence the identity check.
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