I oftentimes see answers using strict comparison (===) instead of normal comparison (==) on status checking, i.e. here:
if(document.readyState === 'complete') ...
I would understand the reason if it were applied on empty string and the obtained value could be also other falsy value with different meaning. But when applied on non-empty string (like 'complete' in the sample), I believe the result is always the same for '==' and '==='. Is that so?
Some people measured that '===' can be faster, but I haven't seen a real world example where it would make any observable difference, so I don't take this micro-optimalization seriously.
On the other hand, anytime I see this operator, I read it as a warning "mind the type here!". But since document.readyState is always string, it annoys me that the original coder made me to study the code what other types there can appear - only to find out that only string.
To me, it is a strong reason to be polite to those who read my code and never use '===' when the type plays no role in the comparison.
Since '===' appears in similar cases in many SO answers and many expert pages, I would like to know if it is just social bandwagon or if there is any good reason why to use it in status checking.
I would recommend always using '===' instead of '==' when strict equality checking is required, which it is in most cases, for this reason: it declares intent. When I see code with '===', I will read it as 'a must be referentially or primitively equal to b'. When I see '==', I will read it as 'a must be coercibly equal to b'. From that, I will judge what kind of goal the code / original programmer is trying to accomplish and how they are passing data around to get the job done. Essentially, it yields insight into the context of the application, the way data is being passed around, and how this function / method / code block fits into the picture.
With that being said, if I see someone do 'a == b' when they are both strings, I'm not going to get on any high horse and make a fuss about it.