JSFiddle - React, Tailwind, and code Playground

HTML

<input type=tel name=contact[phone] id=phone required pattern=^\d+$>

JavaScript

var input = document.getElementsByName('contact[phone]')[0];


input.addEventListener('change', function() {
        console.log('Is valid?',  input.value.search(new RegExp(input.getAttribute('pattern'))) !== -1);
}, false);