JSFiddle - React, Tailwind, and code Playground

HTML

<input>

JavaScript

(function() {
    var elem = $('input');
    var value = elem.val();
    
    elem.bind("input propertychange", function(e) {
        if (elem.val().split('a').length - 1 > 1)
            elem.val(value);
        else
            value = elem.val();
    });
})();