在jQuery样式指南中输入undefined undefined

Question

jQuery has a style guide here: http://contribute.jquery.org/style-guide/js/#type-checks

Type checking undefined varies with scope:

undefined:

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

Why is it necessary and/or preferred to vary this type check?

Answer 1

typeof x can not throw ReferenceError if there is no variable called x . If you try to typeof Boolaen === 'undefined' (typo is intentional) it will return true , if you try Boolaen === undefined it will throw. You can check it in your browsers console.

Further reference: ES 5.1, 11.4.3 - typeof operator

Answer 2

If a global variable hasn't been declared, doing variable === undefined would throw an error ("variable is not defined"). With local variables you know that they're declared so you don't need to take that possibility into account.