Great tool!

Just a warning: If the user has a custom active configuration (like say other accounts, keys, or waits) that will be overwritten or lost when this is pushed:

let auth = {
                threshold: 1,
                keys: [
                    {
                        key: new_public_key,
                        weight: 1
                    }
                ],
                accounts: [],
                waits: []
            };

Eos overwrites the entire auth structure. If you need to merge something you need to read the auth on-line then modify it with the new key. That is more advanced, so a simple check or warning would probably be the way to go.

Also, in that first version of eosjs, if you add forceActionDataHex: false, in cold storage it will create the transaction "data" so in json instead of hex so you can read it.

So when your copying and pasting the transaction back online you know what that "data" line is going to do. The blockproducer will re-serilize the json in "data" and it has to match the transaction.

let EOS = Eos({
   keyProvider: private_key,
   forceActionDataHex: false
   ...

The reason why forceActionDataHex was not on by default, is there is a uncommon case that trigged a de-serialization bug in nodeosd .. You will always be safe with a update transaction. Wost case is the transaction will fail.. I don't recall what exactly triggers it, I could probably look it up if you need to know; it is hard to explain though I have been trying to get it fixed ;) ..

I just wrote a tool too! There is a simple css style sheet in it, I modified from the material-ui .. Feel free to check it out: Bip39 Phrase Chooser.

Also, I have a console eosjs playground .. Nothing fancy but I think you may find lots of hidden treasures within: https://jcalfee.github.io/eos.html

I put my contact info in the footer.. Look me up on telegram, lets build something!