Just realized that a lot of people are modifying instance.defaults.headers by reference.

In this case above PR won't fix the issue to full extent, result sent by adapters make result won't be fully predictable

IMHO modification of configuration (headers in particular) by reference doesn't seem right. Better place may be transformRequest.

But pure functions look like a cleaner approach -- do not allow to mutate anything, require always to produce new object in interceptors, transformers and freeze defaults in DEV mode

@betalb I think the same about cleaner approach, but some people (as with the rest you wrote) needs modify instance.defaults.headers by reference. If we would like to merge this pull request, please also respectively update version of axios and describe this "inconvenience" for mutating programmers ;)

I'm not a huge fan of introducing a new class for this purpose, it seems too bloaty for the codebase. I think it would be best to convert all header names to lowercase at all times, which would remove the need for any special logic related to this.

I'm not closely following current development effort, but remember that there was talk about reworking way how configuration is done. Especially default configuration.

I.e. at the time of writhing, there was a way to directly mutate default headers, inherited by all instances that means by the time we receive headers in adapter we already may have ambiguity.

Note that my PR also doesn't address this issue

The config situation was addressed a while ago (#1395) and it shouldn't be possible to mutate defaults to instances after they have been set. However, you make a good point which I did not initially consider.

Hi,

Thank you for this pull request, this is a pretty stale pull request so I am going to close this in favour of #2880 as its a bit fresher and seems to solve the asme issue. If you feel it does not solve this issue please open an issue again.

Thanks