NOTE: This is not part of the changes request. I am just offering suggestions and we can do it in a separate PR. I can also work on it after this PR get merged.

Great work here.
I like the simplicity, but I could offer an alternative based on fact that the second part is country code addition i.e:

• Bank code (A-Z) : 4 letter code.

Country code (A-Z) : 2 letter code.

• Location Code (0-9 or A-Z) : 2 digit code – either letters or numbers.
• Branch Code (0-9 or A-Z) : optional 3 digit code – either letters or numbers*.

-> We could leverage the array in validISO31661Alpha2CountriesCodes for making this a robust validator.

NB: [Editing code based on comments]

// Making validISO31661Alpha2CountriesCodes exportable or just add to new util file
import { validISO31661Alpha2CountriesCodes } from './isISO31661Alpha2';
//... other code
export default function isBIC(str) {
  assertString(str);
  const countryCode = str.slice(4, 6);
  if(
    validISO31661Alpha2CountriesCodes.indexOf(countryCode) === -1 || 
    countryCode.toUpperCase !== 'XK' // for Republic of Kosovo
   ) {
    return false;
  }
  return isBICReg.test(str);
}

CC. @tux-tn and @profnandaa

i'm happy to make a change doing this - i think it's a good idea. i would just return false without executing the isBICReg.test(str) if it's not a country code - no reason to execute the test if we already know the answer.

it's not clear to me why using includes is useful though; is there a compatibility issue somewhere that i'm not aware of? this seems like it should be a straight, simple use of indexOf() - some has to call a function each time but even then why replace some() with includes() to does the exact same thing but with an extra layer of a function call.

That is great. And I am happy that you are willing to make these changes.

First, I would like to say that, the code above was a sample and required more modifications.

1. If it not country code, please do that return as you have mentioned
2. Take the array of validISO31661Alpha2CountriesCodes from isISO31661Alpha2 and create a new file inside lib/util. Import for both isISO31661Alpha2 and BIC validators.
3. You can make use of indexOf() instead of includes() and some()

I am happy to review that and give a go-ahead to merge this PR.
Thanks again for your contributions @bmacnaughton.

i think this is ready to be merged. it made sense to check that the bank ID was valid.