index_demo.html

<!DOCTYPE html>
<html>
<head>
  <title>Hello world</title>
</head>
<body>
<h1>Currency Facts</h1>

<input id="currency" type="text" />
<input id="amount" type="text" />
<button id="submit" type="button">Submit</button>

<p id="contents"></p>

<script type="text/javascript">


var currency = document.getElementById('currency');
var amount = document.getElementById('amount');

  document.getElementById('submit').addEventListener('click', function (evt) {
    var url = `/currencies/${currency.value}?value=${amount.value}`;

    var input = document.getElementById('currency');
    var country = input.value;
    fetch(url).then(function (res) {
      return res.json().then(function (contents) {
        var string = `${amount.value} USD = ${contents.converted} ${contents.currency}`;
        document.getElementById('contents').innerHTML = string;
      });
    });
  });
</script>
</body>
</html>