javascript – is there a difference between env / ENV and PORT / port in node js

Answering your questions:

whether all of the above are valid.
no, they are not. process.ENV is not the same as process.env. In fact the built-in process does not have an ENV property; so it is not valid unless you create a process.ENV property (wichi is a very bad idea btw) But in windows process.env.PORT and process.env.port they are interchangeably valid

Why are they valid ? because eventhough JS is case sensitive, the standard process.env property is, under windows, for cross-platform compatibility.

is there a difference between any of them ? .env and .ENV are different totally different .ENV does not even exist; on windows between .port and .PORT there is not any practical difference.