JSFiddle - React, Tailwind, and code Playground

HTML

<button id="myBtn" class="btn">Add Entry</button>
<div id="myForm"></div>
<br/>
<button id="btnGraph" class="btn" disabled="true">Graph</button>

JavaScript

$(document).ready(function()
    {
    $("#myBtn").click(function()
    {
        addCountry();

    });

    $(document).on("keyup blur", "input[class^='text']", function(){
         var empty = false;
         $("input[class^='text']").filter(function()
         {
                if($(this).val().trim()=='')
                   empty = true;      
         });


         $('#btnGraph').prop('disabled', empty);
    });

  });

var counter = 1;
var str = '';

function addCountry() 
{
	str = ''+counter+'. Country <input type="text" class="text" id="country-'+counter+'" onkeyup="checkCountry(this)"> Population <input type="text" class="text" id="population-'+counter+'" onkeyup="checkPopulation(this)"><br/>';
	$("#myForm").append(str);
	counter++;
}

function checkPopulation(ob) 
{
	var invalidChars = /[^0-9]/gi
	
	if(invalidChars.test(ob.value)) 
	{
		ob.value = ob.value.replace(invalidChars,"");
	}
}

function checkCountry(ob) 
{
	var invalidChars = /[^a-z]/gi
  
	if(invalidChars.test(ob.value)) 
	{
		ob.value = ob.value.replace(invalidChars,"");
	}
}