JSFiddle - React, Tailwind, and code Playground

HTML

<form onsubmit="return check()">
    <label>Donne un chiffre</label>
    <input type="tel" required="" pattern="[0-9]+" id="nombre"/>
    <input type="submit"/>
</form>

JavaScript

function check()
{
    if (isNaN($('#nombre').val()))
        alert('Devrait pas arriver');
    else
        alert('Ok');
    return false;
}