[Vlasenko Fedor]

<input is="input-separator" id="test" value=""/>
<button onclick="alert(test.value)">Show value</button>
<script>
    customElements.define('input-separator', class extends HTMLInputElement {
        constructor() {
            super();
            addEventListener('input', _ => {
                this.value = new Intl.NumberFormat('en-GB').format(this.value.replace(/\D+/g, ''));
            })
        }
    }, {extends: 'input'});
</script>

вариант