JSFiddle - React, Tailwind, and code Playground

HTML

<td>3.3</td>

<select id="currency1">
  <option value="EUR">Euro</option>
  <option value="USD">US Dollar</option>
  <option value="GBP">British Pound</option>
  <option value="AUD">Australian Dollar</option>
  <option value="JPY">Japanese Yen</option>
</select>
<input type="number" id="cur1">
<button id="btnConvert">Convert</button>
<select id="currency2">
  <option value="EUR">Euro</option>
  <option value="USD">US Dollar</option>
  <option value="GBP">British Pound</option>
  <option value="AUD">Australian Dollar</option>
  <option value="JPY">Japanese Yen</option>
</select>
<input type="text" id="cur2">

JavaScript

window.onload = function() {
  document.getElementById("btnConvert").addEventListener("click", convert);
}

function convert() {

  var select = document.getElementById("currency1");
  var curr = select.options[select.selectedIndex].value;
  var val = document.getElementById("cur2").value;
  if (curr == "EUR") {
    document.getElementById("cur2").value = EURtoOther();
  }
  if (curr == "USD") {
    document.getElementById("cur2").value = USDtoOther();
  }
  if (curr == "GBP") {
    document.getElementById("cur2").value = GBPtoOther();
  }
  if (curr == "AUD") {
    document.getElementById("cur2").value = AUDtoOther();
  }
  if (curr == "JPY") {
    document.getElementById("cur2").value = JPYtoOther();
  }
  
}

function EURtoOther() {
  var select = document.getElementById("currency2");
  var curr = select.value;
  var initval = document.getElementById("cur1").value;
  if (curr == "EUR") {
    return initval * 1.0;
  }
  if (curr == "USD") {
    return initval * 1.13;
  }
  if (curr == "GBP") {
    return initval * 0.79;
  }
  if (curr == "AUD") {
    return initval * 1.55;
  }
  if (curr == "JPY") {
    return initval * 123.27;
  }
}