TLDR: These functions btoa and atob are deprecated for Node JS. If you are coding for the web browser you just need to prepend the window to get rid of this deprecation mark.

to encode :

window.btoa('test') 

result: dGVzdA==

to decode

window.atob('dGVzdA==')

result: test

Here is a React sample using javascript module to browser, but it can run on any modern javascript frontend app that uses webpack or parcel or even vanilla javascript with script src:

//this will run on browser    
import React from "react";
import { Buffer } from 'buffer';
export default function App() {

  const encodeBase64 = (data) => {
    return Buffer.from(data).toString('base64');
  }
  const decodeBase64 = (data) => {
    return Buffer.from(data, 'base64').toString('ascii');
  }

  return <div>
    {'encoded test to base64 = ' + encodeBase64('test')}<br />
    {'decoded dGVzdA== to ascII = ' + decodeBase64('dGVzdA==')}
  </div>;
}

The above react hook will result:

encoded test to base64 = dGVzdA==
decoded dGVzdA== to ascII = test

btoa(): accepts a string where each character represents an 8bit byte. If you pass a string containing characters that cannot be represented in 8 bits, it will probably break. Probably that's why btoa is deprecated.

atob(): returns a string where each character represents an 8-bit byte.

If you are using nodejs just replace atob and btoa with Buffer. Here is the official documentation Buffers in nodejs:

//With NodeJS

export const encodeBase64 = (data) => {
    return Buffer.from(data).toString('base64');
}
export const decodeBase64 = (data) => {
    return Buffer.from(data, 'base64').toString('ascii');
}