Is there any differences between
var a;
(a == undefined)
(a === undefined)
((typeof a) == "undefined")
((typeof a) === "undefined")
Which one should we use?
See Question&Answers more detail:osIs there any differences between
var a;
(a == undefined)
(a === undefined)
((typeof a) == "undefined")
((typeof a) === "undefined")
Which one should we use?
See Question&Answers more detail:osIronically, undefined
can be redefined in JavaScript, not that anyone in their right mind would do that, for example:
undefined = "LOL!";
at which point all future equality checks against undefined
will yeild unexpected results!
As for the difference between ==
and ===
(the equality operators), == will attempt to coerce values from one type to another, in English that means that 0 == "0"
will evaluate to true even though the types differ (Number vs String) - developers tend to avoid this type of loose equality as it can lead to difficult to debug errors in your code.
As a result it's safest to use:
"undefined" === typeof a
When checking for undefinedness :)