JSFiddle - React, Tailwind, and code Playground

HTML

<textarea id="txt"></textarea>

JavaScript

$('#txt').on('keypress', function (e) {
    var pressedKey = String.fromCharCode(e.keyCode);
    var $txt = $('#txt');
    e.preventDefault();
    if (pressedKey == 'a') {
        $txt.val($txt.val() + 'z');
    }
});