Question

Is there any differences between

var a;
(a == undefined)
(a === undefined)
((typeof a) == "undefined")
((typeof a) === "undefined")

Which one should we use?

Answer 1

Ironically, 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 🙂