Is there some description of transaction extra field? From rpc-request to blockchain I have tx object with extra(Uint8Array). In monero repo I found how they parse extra field, but don't understand in details. I need to extract transaction public key to select my self transactions and calculate balance. Then I hope to find some way to create spending transaction.
The txextra field specification is here: https://cryptonote.org/cns/cns005.txt
That document references "varints", which are documented in section 1.2 here: https://tukaani.org/xz/xz-file-format.txt
You should read through all txextra sub-fields until you arrive at the field with tag 0x01, and the 32 byte tx public key will follow.