coins.html

<label for="euroinput">€</label>
<input name="euro" id="euroinput" type="number" min="0.05" step="0.05"></input>
<p id="coins"></p>
<script>
  const euroCoins = [0.05, 0.1, 0.2, 0.5, 1, 2, 5, 10, 20, 50, 100, 200, 500];

  function toCoins(amount, coins) {
    coins.sort((a,b)=>{
      return a===b?0:a<b?1:-1;
    });
    return coins
      .map(coinValue => {
        let coinAmount = 0;
        while (amount>=coinValue) {
          amount -= coinValue;
          coinAmount++;
        }
        return [coinValue, coinAmount];
      })
      .filter(coin=>coin[1]!==0);
  }

  const coinsToString = coins => coins.reduce((acc, [coinValue, coinAmount])=>`${acc}${coinAmount}x ${coinValue>=1?coinValue:coinValue*100} ${coinValue>=1?'Euro':'Cent'}\n`, '');

  const euroInput = document.getElementById('euroinput');
  euroInput.addEventListener('input',  => {
    document.getElementById('coins').innerText = coinsToString(toCoins(parseFloat(euroInput.value.replace(',','.')), euroCoins));
  });
</script>