Incorrect types for some functions?

[ibarrae]

First of all, thanks for the library has been really helpful :tada:.

On a project I have been working, we have found out that at least getAlpha3Code and alpha2ToAlpha3 return types are undefined | string even though in index.d.ts both return types are string.

[michaelwittig]

You are right. Do you feel comfortable providing a fix?

[ibarrae]

Sure, I can work on a fix in the upcoming days :+1:. But should that be on top of #164 ? I just figured out there's a PR for this :thinking:

[michaelwittig]

PR in #198