I am trying to find information about this JavaScript/node.js function:
HMACSHA256()
Its mentioned e.g. here:
https://jwt.io/introduction (scroll to section "Signature")
https://hirosht.medium.com/customized-jwt-encoded-with-hmac-sha256-hs256-267574bd7639 (scroll to section "Signature;")
It's mentioned in other places too.
Neither node.js nor any browser JavaScript provides any function with that name.
So what is it? Is it pseudo code or is it a dummy function or simply a mistake everybody copies and pastes?
You need a plugin named "crypto" It has "createHmac" function where you can give "sha256" as an argument.
https://nodejs.org/api/crypto.html#cryptocreatehmacalgorithm-key-options
It is a standard algorithum.
The JWT documentation is providing psuedo-code (and also links off to JWT specific libraries that you should probably be using instead of writing your own implementation).
The blog you've found has code examples using Crypto-JS