typeof a == 'undefined' vs typeof a === 'undefined'

Question

As I understand the prefered way to check undefined variables is typeof a === 'undefined'.

But why it is better then typeof a == 'undefined'? In which places can it fail?

Answer 1

In this instance, since typeof will always give you a string: It isn't better (nor is it worse). It makes no practical difference.

In general, using === is preferred because it forces you to be explicit about your types and saves you from getting results you don't expect when JavaScript's type resolution rules are unintuitive.

Answer 2

The difference between == and === is that == performs conversions. So for instance 1 will be == to '1' but not === to '1'. The reason why that approach is preferred when you check for undefined is because in JavaScript there are known comparison pitfalls.

The most common:

''        ==   '0'           //false
0         ==   ''            //true
0         ==   '0'           //true
false     ==   'false'       //false
false     ==   '0'           //true
false     ==   undefined     //false
false     ==   null          //false
null      ==   undefined     //true
" \t\r\n" ==   0             //true

So with === you are avoiding the null == undefined problem, which can lead to hard-to-find bugs. That's why you should use == instead of ===. Because === is not performing any conversions behind the scenes, it is also a faster operation.

In this specific case, it won't make a difference effect-wise. Whether you use typeof a == 'undefined' or typeof a === 'undefined' the output will be the same, with no bugs. That's because typeof returns a string. However, the operation will be faster, so you have a negligible performance increase.