Cacher is the code snippet organizer for pro developers

$result = $( '#result' );

var render = function () {

    // レートの計算式
    var jpy = model.getAmount() / vsJPY[ 'JPY' ] * vsJPY[ model.getCurrency() ];
    var usd = model.getAmount() / vsJPY[ 'USD' ] * vsJPY[ model.getCurrency() ];
    var eur = model.getAmount() / vsJPY[ 'EUR' ] * vsJPY[ model.getCurrency() ];
    var gbp = model.getAmount() / vsJPY[ 'GBP' ] * vsJPY[ model.getCurrency() ];

    // 計算結果をHTMLにして出力
    var htmlSrc =
    '<table>' +
        '<caption>両替後</caption>' +
        '<tr><th>￥</th><td>' + jpy + ' 円</td></tr>' +
        '<tr><th>$</th><td>' + usd + ' ドル</td></tr>' +
        '<tr><th>€</th><td>' + eur + ' ユーロ</td></tr>' +
        '<tr><th>£</th><td>' + gbp + ' ポンド</td></tr>' +
    '</table>';

    $result.html( htmlSrc );

}

// 初期表示用に一度描画しておく
render();
// 操作ごとdataChangeイベントを受け取り、再描画する
model.dispatcher.on( 'dataChange', render );

var model = {

  dispatcher: $( {} ),

  _amount  : 100,
  _currency: 'JPY',

  getAmount: function () {

    return this._amount;

  },

  setAmount: function ( val ) {

    this._amount = val;
    this.dispatcher.trigger( 'dataChange' );

  },

  getCurrency: function () {

    return this._currency;

  },

  setCurrency: function ( val ) {

    this._currency = val;
    this.dispatcher.trigger( 'dataChange' );

  }

};

// 値の計算補助用のレート
var vsJPY = {
  'JPY': 1,   // 1円     → 1円
  'USD': 110, // 1ドル   → 110円
  'EUR': 130, // 1ユーロ → 130円
  'GBP': 150  // 1ポンド → 150円
};

<input type="number" id="amount" value="100">
<select id="currency">
    <option value="JPY">円</option>
    <option value="USD">ドル</option>
    <option value="EUR">ユーロ</option>
    <option value="GBP">ポンド</option>
</select>

<div id="result"></div>

var $amount = $( '#amount' );     // input要素
var $currency = $( '#currency' ); // select要素

$amount.on( 'input', function () {

  model.setAmount( parseInt( $amount.val() ) );

} );

$amount.on( 'change', function () {

  model.setAmount( parseInt( $amount.val() ) );

} );

$currency.on( 'change', function () {

  model.setCurrency( $currency.val() );

} );