How to add custom local JS files in Gatsby correctly

I´m a newbie on react and gatsby but i´m working on a little project as practice anda I have a little problem. I want to add a custom JS file to the project (little functions for a calculator on the index). I used Helmet to import them and on develop enviroment is working fine, but once build, is not.

import Helmet from "react-helmet"
import { withPrefix, Link } from "gatsby"

export default function homePage() {
  return (
    <main>
      <Helmet>
        <script src={withPrefix('/functions.js')} type="text/javascript" />
        <script src={withPrefix('/escritura.js')} type="text/javascript" />
      </Helmet>
}

I´m not sure what I am doing wrong. Someone can help me, please? You can see the project proof version live here:

https://modest-hoover-aac2d1.netlify.app/

In the final version, every input should be filled automatically, but is not happening.