JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" id="idtelf" value=2189>

JavaScript

numberValidate("idtelf")

function numberValidate(idtelf){
    var input = document.getElementById('idtelf');

    var number = input.value.toString();
	var myString = number.replace(/\D/g,'');
    if(number.length ==4 && number.length == myString.length)
    {
        alert(  "Correct");
    }
    else {
            alert(  "Enter a valid number");
       }
};