javascript - 如何在两个之间交换选项

function swap() {
  let inp = document.getElementById("inputCurrency").value;
  let out = document.getElementById("outputCurrency").value;
  document.getElementById("inputCurrency").value = out;
  document.getElementById("outputCurrency").value = inp;
}

let amountInput = document.getElementById("amount");

let inputCurrency = document.getElementById("inputCurrency");
let outputCurrency = document.getElementById("outputCurrency");

let convertButton = document.getElementById('convertButton');
//convertButton.addEventListener("click",convertCurrency);

let exchangeButton = document.getElementById("exchange");
exchangeButton.addEventListener("click",swap);

<select id="inputCurrency">
<option value="1">USD</option>
<option value="2">KRW</option>
</select>
<select id="outputCurrency">
<option value="1">USD</option>
<option value="2">KRW</option>
</select>
<input type="text" id="amount"/>
<div class="form-group col-4">
    <button type="button" class="btn bg-dark text-white" id="exchange" style="margin-top: 23px; margin-left: 10px;"> Exchange
          <i class="fa fa-exchange" aria-hidden="true"></i>
    </button>
</div>
<button id="convertButton">Convert</button>