JSFiddle - React, Tailwind, and code Playground

HTML

<input id="a" class="textbox" type="text" name="abc" size="40" />

<input class="btn" type="button" id="add_a" value="ADD" disabled="true" />
<p id="demo"></p>

JavaScript

$(document).on("keyup", "#a", function() {
		
    var originalText = $("#add_a").val();
    var textTextBox = $("#a").val()
    var lengthTextBox = $("#a").val().length;
    
    
    
    if(lengthTextBox > 0){
    	
    	if(originalText !== (textTextBox)){
      	alert(lengthTextBox)
      	$("#add_a").prop("disabled", false);
      }else{
      	$("#add_a").prop("disabled", true);
      }
    }else{
    	$("#add_a").prop("disabled", true);
    }
    
    
    /*var emptyInputs = $("#a").filter(function() {
        return $.trim( $(this).val() ) == ""
    }).length;
    $("#add_a").prop("disabled", emptyInputs);*/
});