Best script/pseudocode to convert xpriv to xpub? I'm looking for a script/pseudocode/utility (preferably, code in javascript/c++ or a secure, verified and open-source rpc client) that can convert a given xpriv to xpub.

Only one I've found (but not tried yet) is this one  :
https://github.com/sarchar/brainwallet.github.com/tree/bip32

or I know electrum wallet client can do it. I would prefer to read through the code of a standalone script though and/or create my own script
If you are comfortable with using the command line then libbitcoin-explorer is a great utility for converting your bitcoin HD keys. It even works completely offline. 

Once you have bx installed, you can run the hd-to-public command like so:

$ bx hd-to-public xprv9s21ZrQH143K3QTDL4LXw2F7HEK3wJUD2nW2nRk4stbPy6cq3jPPqjiChkVvvNKmPGJxWUtg6LnF5kejMRNNU3TGtRBeJgk33yuGBxrMPHi
=&gt; xpub661MyMwAqRbcFtXgS5sYJABqqG9YLmC4Q1Rdap9gSE8NqtwybGhePY2gZ29ESFjqJoCu1Rupje8YtGqsefD265TMg7usUDFdp6W1EGMcet8


More info about the other HD Key related commands for libbitcoin-explorer can be found here: https://github.com/libbitcoin/libbitcoin-explorer/wiki/bx-hd-to-public