How to handle backspace with libphonenumber AsYouTypeFormatter

Question

I'm trying to use google-libphonenumber's AsYouTypeFormatter with a simple input element on a web form. I pass each key typed by the user to the inputDigit method. The problem I'm running into is that when the user hits backspace, google-libphonenumber doesn't remove the last digit and simply appends Backspace to the phone number. Am I using the AsYouTypeFormatter improperly? Is it not able to deal with backspaces? If so, and I suspect that is the case, how should I handle the case where the user presses backspace?

Here is a link to an example project: https://stackblitz.com/edit/libphonenumber

And here is the code:

import { AsYouTypeFormatter } from 'google-libphonenumber';

const appDiv: HTMLElement = document.getElementById('app');
appDiv.innerHTML = `
  <h1>Libphonenumber Playground</h1>
  <input id="input" type="text">
`;

this.formatter = new AsYouTypeFormatter('us');

const input = document.getElementById('input') as HTMLInputElement;

input.addEventListener('keyup', (event: KeyboardEvent) => {
  console.log(this.formatter.inputDigit(event.key));
});

Answer 1

"isNumber": function(evt) {
    evt = (evt) ? evt : window.event;
    var charCode = (evt.which) ? evt.which : evt.keyCode;

if ((charCode > 31 && (charCode < 48 || charCode > 57) || charCode === 46 || charCode === 13)) {
    //do not allow non numeric characters 
    evt.preventDefault();
}else{
    //ok: numeric character
    //handle phone number masking.
    this.phoneNumberForm =this.formatter.inputDigit(evt.key);
}

I haven't found any methods that will move the formatter index back. (That doesn't mean that they are not there) What I did was prevent the backspace keydown event. Along with any other non-numeric character. Here is the function I used: