JSFiddle - React, Tailwind, and code Playground

HTML

<input id='someId' type='text'>
<p id='result'>
</p>

JavaScript

function check(e,element) {
    var acceptable = 'abcd12';
    var char = element.value.charAt(element.value.length - 1);
    if(acceptable.indexOf(char)>=0){
        document.getElementById("result").innerHTML="true";
    }
    else
    {
        document.getElementById("result").innerHTML="false";
    }
}

var inputElement = document.getElementById('someId');
inputElement.addEventListener('keyup', function( e ) {
                check( e, inputElement )
});