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HTML 

              
                <body>
    <img src="https://i.imgur.com/swbDFKF.png" alt="money image" class="money-img" />
    <h1>Exchange Rate Calculator</h1>
    <p>Choose the currency and the amounts to get the exchange rate</p>
    <div class="container">
      <!-- currency 01 -->
      <div class="currency">
        <select id="currency-1">
          <option value="AED">AED</option>
          <option value="ARS">ARS</option>
          <option value="AUD">AUD</option>
          <option value="BGN">BGN</option>
          <option value="BRL">BRL</option>
          <option value="BSD">BSD</option>
          <option value="CAD">CAD</option>
          <option value="CHF">CHF</option>
          <option value="CLP">CLP</option>
          <option value="CNY">CNY</option>
          <option value="COP">COP</option>
          <option value="CZK">CZK</option>
          <option value="DKK">DKK</option>
          <option value="DOP">DOP</option>
          <option value="EGP">EGP</option>
          <option value="EUR">EUR</option>
          <option value="FJD">FJD</option>
          <option value="GBP">GBP</option>
          <option value="GTQ">GTQ</option>
          <option value="HKD">HKD</option>
          <option value="HRK">HRK</option>
          <option value="HUF">HUF</option>
          <option value="IDR">IDR</option>
          <option value="ILS">ILS</option>
          <option value="INR">INR</option>
          <option value="ISK">ISK</option>
          <option value="JPY">JPY</option>
          <option value="KRW">KRW</option>
          <option value="KZT">KZT</option>
          <option value="MXN">MXN</option>
          <option value="MYR">MYR</option>
          <option value="NOK">NOK</option>
          <option value="NZD">NZD</option>
          <option value="PAB">PAB</option>
          <option value="PEN">PEN</option>
          <option value="PHP">PHP</option>
          <option value="PKR">PKR</option>
          <option value="PLN">PLN</option>
          <option value="PYG">PYG</option>
          <option value="RON">RON</option>
          <option value="RUB">RUB</option>
          <option value="SAR">SAR</option>
          <option value="SEK">SEK</option>
          <option value="SGD">SGD</option>
          <option value="THB">THB</option>
          <option value="TRY">TRY</option>
          <option value="TWD">TWD</option>
          <option value="UAH">UAH</option>
          <option value="USD" selected>USD</option>
          <option value="UYU">UYU</option>
          <option value="VND">VND</option>
          <option value="ZAR">ZAR</option>
        </select>
        <input type="number" id="amount-one" placeholder="0" value="1" />
      </div>

      <!-- swap buttoh -->
      <div class="swap-container">
        <button class="btn" id="swap">swap</button>
        <div class="rate" id="rate"></div>
      </div>

      <!-- currency 02 -->
      <div class="currency">
        <select id="currency-2">
          <option value="AED">AED</option>
          <option value="ARS">ARS</option>
          <option value="AUD">AUD</option>
          <option value="BGN">BGN</option>
          <option value="BRL">BRL</option>
          <option value="BSD">BSD</option>
          <option value="CAD">CAD</option>
          <option value="CHF">CHF</option>
          <option value="CLP">CLP</option>
          <option value="CNY">CNY</option>
          <option value="COP">COP</option>
          <option value="CZK">CZK</option>
          <option value="DKK">DKK</option>
          <option value="DOP">DOP</option>
          <option value="EGP">EGP</option>
          <option value="EUR">EUR</option>
          <option value="FJD">FJD</option>
          <option value="GBP">GBP</option>
          <option value="GTQ">GTQ</option>
          <option value="HKD">HKD</option>
          <option value="HRK">HRK</option>
          <option value="HUF">HUF</option>
          <option value="IDR">IDR</option>
          <option value="ILS">ILS</option>
          <option value="INR">INR</option>
          <option value="ISK">ISK</option>
          <option value="JPY">JPY</option>
          <option value="KRW">KRW</option>
          <option value="KZT">KZT</option>
          <option value="MXN">MXN</option>
          <option value="MYR">MYR</option>
          <option value="NOK">NOK</option>
          <option value="NZD">NZD</option>
          <option value="PAB">PAB</option>
          <option value="PEN">PEN</option>
          <option value="PHP">PHP</option>
          <option value="PKR">PKR</option>
          <option value="PLN">PLN</option>
          <option value="PYG">PYG</option>
          <option value="RON">RON</option>
          <option value="RUB">RUB</option>
          <option value="SAR">SAR</option>
          <option value="SEK">SEK</option>
          <option value="SGD">SGD</option>
          <option value="THB">THB</option>
          <option value="TRY">TRY</option>
          <option value="TWD">TWD</option>
          <option value="UAH">UAH</option>
          <option value="USD" selected>USD</option>
          <option value="UYU">UYU</option>
          <option value="VND">VND</option>
          <option value="ZAR">ZAR</option>
        </select>
        <input type="number" id="amount-two" placeholder="0" value="1" />
      </div>
    </div>

    <script src="./script.js"></script>
  </body>
!

CSS 

              
                :root {
  --primary-color: #5fbaa7;
}
* {
  box-sizing: border-box;
}

body {
  background-color: #f4f4f4;
  font-family: "Segoe UI", Tahoma, Geneva, Verdana, sans-serif;
  display: flex;
  flex-direction: column;
  justify-content: center;
  align-items: center;
  height: 100vh;
  padding: 10px;
  margin: 0;
}
.money-img {
  width: 150px;
}

h1 {
  color: var(--primary-color);
  text-align: center;
  margin: 5px 0 0 0;
  /* color: blue; */
}

p {
  text-align: center;
  margin: 5px 0 0 0;
}

.btn {
  color: #fff;
  background-color: var(--primary-color);
  cursor: pointer;
  font-size: 12px;
  border-radius: 5px;
  padding: 5px 12px;
  border: none;
}

.currency {
  padding: 40px 0;
  display: flex;
  justify-content: space-between;
  align-items: center;
}

.currency select {
  padding: 10px 20px 5px 10px;
  -moz-appearance: none;
  -webkit-appearance: none;
  appearance: none;
  border: 1px solid #dedede;
  background: transparent;
  font-size: 16px;
  background-image: url("data:image/svg+xml;charset=US-ASCII,%3Csvg%20xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg%22%20width%3D%22292.4%22%20height%3D%22292.4%22%3E%3Cpath%20fill%3D%22%20000002%22%20d%3D%22M287%2069.4a17.6%2017.6%200%200%200-13-5.4H18.4c-5%200-9.3%201.8-12.9%205.4A17.6%2017.6%200%200%200%200%2082.2c0%205%201.8%209.3%205.4%2012.9l128%20127.9c3.6%203.6%207.8%205.4%2012.8%205.4s9.2-1.8%2012.8-5.4L287%2095c3.5-3.5%205.4-7.8%205.4-12.8%200-5-1.9-9.2-5.5-12.8z%22%2F%3E%3C%2Fsvg%3E");
  background-position: right 10px top 50%, 0, 0;
  background-size: 12px auto, 100%;
  background-repeat: no-repeat;
}

.currency input {
  border: 0;
  background: transparent;
  font-size: 30px;
  text-align: right;
}

.swap-container {
  display: flex;
  justify-content: space-between;
  align-items: center;
}

.rate {
  color: var(--primary-color);
  font-size: 14px;
  padding: 0 10px;
}

select:focus,
input:focus,
button:focus {
  outline: 0;
}

@media (max-width: 600px) {
  .currency input {
    width: 200px;
  }
}
!

JS 

              
                const currency_1 = document.getElementById("currency-1");
const amountOne = document.getElementById("amount-one");
const currency_2 = document.getElementById("currency-2");
const amountTwo = document.getElementById("amount-two");

const rateE1 = document.getElementById("rate");
const swap = document.getElementById("swap");

//Calculate Rates
function calculate() {
  let currency_One = currency_1.value;
  let currency_Two = currency_2.value;

  fetch(`https://api.exchangerate-api.com/v4/latest/${currency_One}`)
    .then((res) => res.json())
    .then((data) => {
      const rate = data.rates[currency_Two];

      rateE1.innerHTML = `1 ${currency_One} = ${rate} ${currency_Two} `;
      amountTwo.value = (rate * amountOne.value).toFixed(2);
    });
}

//Event Listensers
currency_1.addEventListener("change", calculate);
amountOne.addEventListener("input", calculate);
currency_2.addEventListener("change", calculate);
amountTwo.addEventListener("input", calculate);

swap.addEventListener("click", () => {
  const temp = currency_1.value;
  currency_1.value = currency_2.value;
  currency_2.value = temp;

  amountOne.value = amountTwo.value;
  calculate();
});

calculate();
!
999px

Console