JavaScript：typeof(var)==='undefined' 与 =='undefined' 之间的区别？

Question

Perhaps a somewhat theoretical question, but to check if a variable exists or not, this is most commonly advised:

typeof(var)==='undefined' or typeof(var)!=='undefined'

How does this differ from typeof(var)=='undefined' (or typeof(var)!='undefined' ) ?

I mean === vs == . Or !== vs != . I know this normally means comparison of type as well as value, but in this case, typeof(something) always evaluates to a string, right?

Is there any scenario possible where typeof(var)==='undefined' and typeof(var)=='undefined' are not the same?

Answer 1

There is really no difference thus typeof returns a string.

Use === and !== when you want to avoid automatic conversions.

Examples:

alert(1!='1')//false
alert(1!=='1')//true
alert('1'!='1')//false
alert('1'!=='1')//false
alert(true==1)//true
alert(true===1)//false

Answer 2

'===' 和 '!==' 运算符要快一些，因此在大多数情况下，应该使用它们代替 '==' 或 '!='。

Answer 3

As for the != part, the ! is to be taken as a NOT. typeof var !=='undefined' => if typeof var is not undefined