I will add a private key validation as soon as I find time for this. The Steem.js lib offers a wifIsValid( privKey, pubKey ) method to check against a public key, so it's really easy to implement.

is Steemjs an official library?

Yes, it's being maintained by Steemit and the source can be found on GitHub. I think it is still the mostly used JavaScript library for accessing the Steem blockchain, but there are already better/newer alternatives available. (for example DSteem)