JSFiddle - React, Tailwind, and code Playground

by Geoffrey Gazet

HTML

<input type="text" id="champ1"/>
<input type="text" id="champ2"/>

JavaScript

$("#champ2").on('keydown', function() {
    var valCh1 = $("#champ1").val();
    var valCh2 = $(this).val();
    var lengthCh2 = valCh2.length;
    
    if(valCh1.substr(0, lengthCh2) != valCh2) {
        alert("You entered different letter than first field !");
        valCh2 = valCh2.substr(0, lengthCh2 -1);
        $(this).val(valCh2);
    }
});