Анимации для bottom line

/*******************************************************************************
 *  Initialize the amount for the first currency
 *******************************************************************************/
function initializeAmount() {
  document.getElementById('amount-one').addEventListener('input', refreshAmount);
}

/*******************************************************************************
 *  Initialize the Swap trigger already existing in DOM
 *******************************************************************************/
function initializeSwap() {
  swap.addEventListener('click', () => {

    const selectOne = document.querySelector('#cur1.custom-select');
    const selectTwo = document.querySelector('#cur2.custom-select');
    const amountOne = document.getElementById('amount-one');
    const amountTwo = document.getElementById('amount-two');

    const [valueOne, valueTwo] = [selectOne, selectTwo].map(
      select => select.querySelector('.custom-select__option--select').dataset.value
    );

    //--------------------------------------------------------------------------
    // Dirty trick to avoid problems when swapping currencies on the 2 DDs
    // by finding the currencies not chosen by the two dropdowns
    // and first selecting a third currency on the first dropdown
    // before doing the actual swap
    //--------------------------------------------------------------------------
    const availableCurrencies = [];
    const chosenCurrencies = [valueOne, valueTwo];
    selectOne.querySelectorAll('.custom-select__list .custom-select__option-name').forEach((option) => {
      availableCurrencies.push(option.innerText);
    });
    const notChosenCurrencies = availableCurrencies.filter(chosen => !chosenCurrencies.includes(chosen));

    amountOne.value = amountTwo.value
    selectOne.querySelector(`[data-value="${notChosenCurrencies[0]}"]`).click();
    selectOne.querySelector(`[data-value="${valueTwo}"]`).click();
    selectTwo.querySelector(`[data-value="${valueOne}"]`).click();
    refreshConversionRatesAndAmount();
  });
}

/*******************************************************************************
 *  Initialize the Currency Dropdowns already defined in the DOM 
 *******************************************************************************/
function initializeCurrencyDropdowns() {
  //given the array of all .custom-select
  [...document.querySelectorAll(".custom-select")]
  //map()
  .map(select => {

    //inits dropdown options
    let selected = select.querySelector(".custom-select__option--select");
    if (selected) {
      select.dataset.value = selected.dataset.value;
      const placeholder = select.querySelector(".custom-select__placeholder");
      placeholder.innerHTML = "";
      const pho = selected.cloneNode(true);
      pho.classList.remove("custom-select__option--select");
      placeholder.appendChild(pho);
    }

    //adds click event listener to the list of options
    select.querySelector(".custom-select__list").addEventListener("click", e => {
      //--------------------------------------------------------------------------
      // Logic to hide from the other DD the selected currency in this DD
      //--------------------------------------------------------------------------
      //fetches information about the currently selected dropdown
      const thisDD = e.target.closest('.custom-select');
      const currencyCurrentlySelected = e.target.closest('.custom-select__option').dataset.value;


      //fetches information about the other dropdown
      const otherDD_id = ((thisDD.id == 'cur1' ? 'cur2' : 'cur1'));
      const otherDD = document.getElementById(otherDD_id);

      //removes the displaynone class to all the options in the other dropdown
      otherDD.querySelectorAll('.custom-select__option').forEach((option) => {
        option.classList.remove('display-none');
      });
      //adds the class displaynone to the currently selected currency in the second dropdown    
      otherDD.querySelector(`.custom-select__option[data-value="${currencyCurrentlySelected}"]`)
        .classList.add('display-none');

      //-------------------------------------------------------------------------- 

      let target = e.target.closest(".custom-select__option");
      if (target) {
        let parent = target.closest(".custom-select");
        parent
          .querySelector(".custom-select__option--select")
          .classList.remove("custom-select__option--select");
        target.classList.add("custom-select__option--select");
        let selected = parent.querySelector(".custom-select__option--select");
        parent.dataset.value = selected.dataset.value;
        const placeholder = parent.querySelector(".custom-select__placeholder");
        placeholder.innerHTML = "";
        const pho = selected.cloneNode(true);
        pho.classList.remove("custom-select__option--select");
        placeholder.appendChild(pho);
        target.closest(".custom-select").classList.remove("custom-select--drop");
      }

      //-------------------------------------------------------------------------- 

      refreshConversionRatesAndAmount();
    });

    //adds click event listener to the placeholder
    select.querySelector(".custom-select__placeholder").addEventListener("click", e => {
      let target = e.target.closest(".custom-select__placeholder");
      if (target) {
        target.closest(".custom-select").classList.toggle("custom-select--drop");
      }
    });
  });
}

/*******************************************************************************
 * Refresh the conversion rates and converted amount (and metadata)
 * displayed in the respective labels
 *******************************************************************************/
async function refreshConversionRatesAndAmount() {

  const rates = await fetchConversionRates();

  const currencyOne = getSelectedCurrencyOne();
  const currencyTwo = getSelectedCurrencyTwo();

  const rateOne = rates[currencyOne];
  const rateTwo = rates[currencyTwo];

  const amountOne = document.getElementById('amount-one');
  const amountTwo = document.getElementById('amount-two');

  const rateLabel = document.getElementById('rate');




  const ratio = (1 * rateOne.rate / rateTwo.rate).toFixed(4);
  rateLabel.innerText = `1 ${currencyOne} = ${ratio} ${currencyTwo}`;

  amountTwo.dataset.ratio = ratio;
  const newAmount = (amountOne.value * amountTwo.dataset.ratio).toFixed(2);
  amountTwo.value = (amountOne.value !== '') ? newAmount : '';
}

/*******************************************************************************
 * Refresh the amount only 
 *******************************************************************************/
function refreshAmount() {
  const amountOne = document.getElementById('amount-one');
  const amountTwo = document.getElementById('amount-two');
  const newAmount = (amountOne.value * amountTwo.dataset.ratio).toFixed(2);
  amountTwo.value = (amountOne.value !== '') ? newAmount : '';
}

/*******************************************************************************
 * Fetch and return the conversion rates from a static url
 *******************************************************************************/
async function fetchConversionRates() {
  const url = 'https://bank.gov.ua/NBUStatService/v1/statdirectory/exchange?json';
  const response = await fetch(url);
  const rates = await response.json();
  rates.unshift({
    "txt": "Українська гривня",
    "rate": 1,
    "cc": "UAH"
  });
  const ratesMapped = Object.assign({}, ...rates.map((rate) => ({
    [rate.cc]: rate
  })));
  return ratesMapped;
}

/*******************************************************************************
 * Returns the Currency code selected on the first currency dropdown
 *******************************************************************************/
function getSelectedCurrencyOne() {
  const cur1DD = document.querySelector('#cur1.custom-select');
  return cur1DD.textContent.trim().slice(0, 3);
}

/*******************************************************************************
 * Returns the Currency code selected on the second currency dropdown
 *******************************************************************************/
function getSelectedCurrencyTwo() {
  const cur2DD = document.querySelector('#cur2.custom-select');
  return cur2DD.textContent.trim().slice(0, 3);
}

initializeAmount();
initializeSwap();
initializeCurrencyDropdowns();
refreshConversionRatesAndAmount();

body {
  display: flex;
  justify-content: center;
  align-items: center;
  width: 100%;
  min-height: 100vh;
  margin: 0;
}

.custom-select {
  display: block;
  width: 150px;
  border-bottom: 2px solid rgba(0, 0, 0, 0.12);
  position: relative;
  -webkit-user-select: none;
  -moz-user-select: none;
  -ms-user-select: none;
  user-select: none;
  z-index: 1;
  margin-bottom: 8.7px;
}

.custom-select:hover {
  border-bottom: 2px solid rgba(0, 0, 0, 0.54);
}

.custom-select__list {
  display: block;
  border-radius: 5px;
  background-color: #fff;
  padding: 5px 0;
  box-shadow: 0 5px 10px 0 rgba(0, 0, 0, 0.15);
  position: absolute;
  z-index: 999;
  left: 0;
  top: calc(100% + 12px);
  opacity: 0;
  pointer-events: none;
  transform: translateY(10px);
  transition-property: transform, opacity;
  transition-timing-function: ease-out;
  transition-duration: 0.3s;
  box-sizing: border-box;
}

.custom-select--drop .custom-select__list {
  transform: translateY(0);
  opacity: 1;
  pointer-events: all;
}

.custom-select--drop .custom-select__placeholder::after {
  transform: translateY(-80%) rotateX(360deg);
}

.custom-select__option {
  display: flex;
  justify-content: flex-start;
  align-items: center;
  padding: 0.5em 1.5em;
  box-sizing: border-box;
  cursor: pointer;
  z-index: 0;
}

.custom-select__option-icon {
  display: block;
  flex-shrink: 0;
  border-radius: 5px;
  overflow: hidden;
  margin-right: 10px;
}

.custom-select__option-icon img {
  display: block;
  width: 18px;
  height: 15px;
  max-height: 100%;
  -o-object-fit: cover;
  object-fit: cover;
  margin: 0;
}

.custom-select__option-name {
  display: block;
  width: 100%;
  text-transform: uppercase;
  margin-right: 10px;
}

.custom-select__option-symbol {
  display: block;
  font-weight: bold;
}

.custom-select__option:hover {
  background-color: rgba(0, 0, 0, 0.045);
}

.custom-select__option--select .custom-select__option-name {
  color: #8dc641;
}

.custom-select__placeholder {
  display: block;
  width: 100%;
}

.custom-select__placeholder::after {
  content: '';
  display: block;
  width: 0;
  height: 0;
  border: 6px solid transparent;
  border-top-width: 6px;
  border-bottom: 6px solid #616161;
  position: absolute;
  right: 0;
  top: 50%;
  transform: translateY(-30%) rotateX(180deg);
  transition-property: transform;
  transition-timing-function: ease-out;
  transition-duration: 0.3s;
}

.custom-select__option {
  padding-right: calc(1.5em + 10px);
  position: relative;  
}

.custom-select__placeholder .custom-select__option::before,
.custom-select__placeholder .custom-select__option::after{
  content: '';
  position: absolute; bottom: -2px; 
  width: 0;
  height: 2px;
  background-color: #8dc641;
  transition: width .3s ease;
}

.custom-select__placeholder .custom-select__option::before {
  left: 50%;
}

.custom-select__placeholder .custom-select__option::after {
  right: 50%;
}

.custom-select--drop .custom-select__placeholder .custom-select__option::before,
.custom-select--drop .custom-select__placeholder .custom-select__option::after,
.custom-select__placeholder:hover .custom-select__option::before,
.custom-select__placeholder:hover .custom-select__option::after {
  width: 50%;
}

.custom-select__option:hover {
  background-color: transparent;
}


/* .container {
  
  border: solid 2px #212121;
  border-radius: 10px;
  background-color: white;
  box-sizing: border-box;
  -webkit-box-sizing: border-box;
} */

.header {
  display: flex;
  align-items: center;
}

.indent-right {
  display: flex;
  width: 24px;
  height: 24px;
  background-image: url("https://cdn.privat24.ua/icons/file/ServiceCurrency.svg");
  background-repeat: no-repeat;
  background-position: center center;
  background-size: contain;
  margin-right: 10px !important;
}

.btn {
  display: inline-flex;
  -webkit-box-align: center;
  align-items: center;
  -webkit-box-pack: center;
  justify-content: center;
  position: relative;
  width: 57px;
  height: 44px;
  font-size: 16px;
  line-height: 1.5;
  font-family: 'Open Sans', sans-serif;
  font-weight: 600;
  padding: 8px 16px;
  border-radius: 4px;
  transition-duration: 450ms;
  transition-timing-function: cubic-bezier(0.23, 1, 0.32, 1);
  word-break: normal;
  color: rgba(0, 0, 0, 0.54);
  background: rgb(255, 255, 255);
  box-shadow: rgb(0 0 0 / 12%) 0px 2px 4px 0px, rgb(0 0 0 / 12%) 0px 0px 4px 0px;
  white-space: nowrap;
  border-style: none;
  border-width: 0px;
  border-color: rgb(255, 255, 255);
  margin: 0px;
  cursor: pointer;
}

.btn:hover {
  background-color: lightgrey;
}

.swap-rate-container .btn-arrows {
  transform: rotate(90deg);
  opacity: 0.54;
  transition: opacity 450ms cubic-bezier(0.23, 1, 0.32, 1);
  position: absolute;
  z-index: 0;
  font-family: 'Open Sans', sans-serif;
}

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

input::placeholder {
  font-family: 'Open Sans', sans-serif;
  font-size: 16px;
}

.currency input {
  background: transparent;
  font-size: 16px;
  line-height: 1.5;
  text-align: right;
  font-weight: 400;
  color: rgba(0, 0, 0, 0.87);
  border-style: none;
  border-width: 0px;
  border-radius: 0px;
  background: transparent;
  box-shadow: none;
  outline: none;
  width: 65%;
  font-family: 'Open Sans', sans-serif;
  background: transparent;
  border-bottom: 2px solid rgba(0, 0, 0, 0.12);
  margin-left: 15px
}

.currency input:hover {
  display: block;
  line-height: 1.5;
  font-family: "Open Sans", sans-serif;
  border-bottom: 2px solid rgba(0, 0, 0, 0.54);
}

.currency input:focus {
  border-bottom: 2px solid #8dc641;
  border-bottom-color: transparent;
}


/* .currency input::after {
  transform: scaleX(0);  
  transition: transform 250ms ease-in-out;
}

.currency input::before {
  transform: scaleX(0);  
  transition: transform 250ms ease-in-out;
}

.currency input:hover::after {
  transform: scaleX(1);
}

.currency input:hover::before {
  transform: scaleX(1);
} */

.select-placeholder {
  background: rgb(255, 255, 255);
  color: rgba(0, 0, 0, 0.38);
}

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

.rate {
  color: black;
  font-size: 15px;
  padding: 0 10px;
  background: #E0E0E0;
  line-height: var(--tl-small);
  padding: 8px;
  font-family: 'Open Sans', sans-serif;
  border-radius: 2px;
}

input::-webkit-inner-spin-button {
  display: none !important;
}

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

.display-none {
  display: none;
}





.input-container {
  position: relative;
  width: 65%;
  margin-left: 15px;
}

.input-container input {
  width: 100%;
  margin-left: 0;
}

.input-border {
  position: absolute; bottom: 0; left: 0;
  width: 100%;
  height: 2px;
}

.input-border::before,
.input-border::after{
  content: '';
  position: absolute; top: 0; 
  width: 0;
  height: 100%;
  background-color: #8dc641;
  transition: width .3s ease;
}

.input-border::before {
  left: 50%;
}

.input-border::after {
  right: 50%;
}

.input-container input:hover + .input-border::before,
.input-container input:hover + .input-border::after {
  width: 50%;
}

<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8" />
  <meta name="viewport" content="width=device-width, initial-scale=1.0" />
  <title>Currency Converter</title>
  <link rel="shortcut icon" href="currencies-icon-10.jpg" type="image/x-icon">
  <link rel="stylesheet" href="style.css" />
</head>

<body>

  <div class="container">

    <div class="header">
      <div class="indent-right"></div>
      <b>Конвертор валют</b>
    </div>

    <!-- Currency DD 1 -->
    <div class="currency">
      <div class="custom-select" id="cur1">
        <div class="custom-select__placeholder"></div>        
        <div class="custom-select__list">
          <div class="custom-select__option" data-value="EUR">
            <div class="custom-select__option-icon">
              <img src="https://cdn.privat24.ua/icons/file/EU.svg">
            </div>
            <div class="custom-select__option-name">EUR</div>
            <div class="custom-select__option-symbol">€</div>
          </div>
          <div class="custom-select__option custom-select__option--select" data-value="USD">
            <div class="custom-select__option-icon">
              <img src="https://cdn.privat24.ua/icons/file/US.svg">
            </div>
            <div class="custom-select__option-name">USD</div>
            <div class="custom-select__option-symbol">$</div>
          </div>
          <div class="custom-select__option" data-value="GBP">
            <div class="custom-select__option-icon">
              <img src="https://cdn.privat24.ua/icons/file/GB.svg">
            </div>
            <div class="custom-select__option-name">GBP</div>
            <div class="custom-select__option-symbol">£</div>
          </div>
          <div class="custom-select__option  display-none" data-value="UAH">
            <div class="custom-select__option-icon">
              <img src="https://cdn.privat24.ua/icons/file/UA.svg" />
            </div>
            <div class="custom-select__option-name">UAH</div>
            <div class="custom-select__option-symbol">₴</div>
          </div>
        </div>
      </div>
      <div class="input-container">
        <input type="number" id="amount-one" placeholder="200.00" />
        <span class="input-border"></span>
      </div>
    </div>
    <!-- END -->

    <div class="swap-rate-container">
      <button class="btn" id="swap">
          <div class="btn-arrows">
            <div style="display: flex;">
              <svg
                height="24px" width="24px" version="1.1"
                viewBox="0 0 24 24" xmlns="http://www.w3.org/2000/svg">
                <g fill="none" fill-rule="evenodd" stroke="none" stroke-width="1">
                  <path d="M0 0h24v24H0z"></path>
                  <path
                    d="M6.99 11L3 15l3.99 4v-3H14v-2H6.99v-3zM21 9l-3.99-4v3H10v2h7.01v3L21 9z"
                    fill="#000"
                    fill-rule="nonzero"></path>
                </g>
              </svg>
            </div>
          </div>
        </button>
      <div class="rate" id="rate"></div>
    </div>

    <!-- Currenct DD 2 -->
    <div class="currency">
      <div class="custom-select" id="cur2">
        <div class="custom-select__placeholder"></div>
        <div class="custom-select__list">
          <div class="custom-select__option custom-select__option--select" data-value="UAH">
            <div class="custom-select__option-icon">
              <img src="https://cdn.privat24.ua/icons/file/UA.svg" />
            </div>
            <div class="custom-select__option-name">UAH</div>
            <div class="custom-select__option-symbol">₴</div>
          </div>
          <div class="custom-select__option" data-value="GBP">
            <div class="custom-select__option-icon">
              <img src="https://cdn.privat24.ua/icons/file/GB.svg">
            </div>
            <div class="custom-select__option-name">GBP</div>
            <div class="custom-select__option-symbol">£</div>
          </div>
          <div class="custom-select__option" data-value="EUR">
            <div class="custom-select__option-icon">
              <img src="https://cdn.privat24.ua/icons/file/EU.svg">
            </div>
            <div class="custom-select__option-name">EUR</div>
            <div class="custom-select__option-symbol">€</div>
          </div>
          <div class="custom-select__option display-none" data-value="USD">
            <div class="custom-select__option-icon">
              <img src="https://cdn.privat24.ua/icons/file/US.svg">
            </div>
            <div class="custom-select__option-name">USD</div>
            <div class="custom-select__option-symbol">$</div>
          </div>
        </div>
      </div>
      <div class="input-container">
        <input type="number" id="amount-two" placeholder="200.00" />
        <span class="input-border"></span>
      </div>
    </div>
    <!-- END -->

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

</html>