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HTML

ConversĂŁo de USD em EUR
<section>
    <div>Qual a quantidade?</div>
    <div>
        <input type="text" id="quantidade" />
    </div>
    <div>De:</div>
    <div>
        <select id="de">
            <option value="BRL">Reais Brazileiros - BRL</option>
            <option selected="selected" value="USD">United States Dollars - USD</option>
            <option value="EUR">Euro - EUR</option>
            <option value="GBP">United Kingdom Pounds - GBP</option>
            <option value="CAD">Canada Dollars - CAD</option>
            <option value="AUD">Australia Dollars - AUD</option>
            <option value="JPY">Japan Yen - JPY</option>
            <option value="INR">India Rupees - INR</option>
            <option value="NZD">New Zealand Dollars - NZD</option>
            <option value="CHF">Switzerland Francs - CHF</option>
        </select>
    </div>
    <div>Para:</div>
    <div>
        <select id="para">
            <option value="BRL">Reais Brazileiros - BRL</option>
            <option selected="selected" value="USD">United States Dollars - USD</option>
            <option value="EUR">Euro - EUR</option>
            <option value="GBP">United Kingdom Pounds - GBP</option>
            <option value="CAD">Canada Dollars - CAD</option>
            <option value="AUD">Australia Dollars - AUD</option>
            <option value="JPY">Japan Yen - JPY</option>
            <option value="INR">India Rupees - INR</option>
            <option value="NZD">New Zealand Dollars - NZD</option>
            <option value="CHF">Switzerland Francs - CHF</option>
        </select>
    </div>
    <div>
        <button type="button">Converter</button>
    </div>
    <div id="resultado"></div>
</section>

CSS

div {
    height:30px;
    min-width:50px;
}
div:nth-child(odd) {
    float:left;
}
div:nth-child(even) {
    float:right;
}
section {
    width:300px;
    margin:auto;
}

JavaScript

Request.exchange = new Class({
    Extends: Request.JSONP,
    options: {
        url: 'http://rate-exchange.appspot.com/currency?from={from}&to={to}&q={amount}',
        amount: 1
    },
    initialize: function (options) {
        this.setOptions(options);
        this.options.url = this.options.url.substitute(this.options);
        this.parent();
    }
});
window.addEvent('domready', function () {
    document.getElement('button').addEvent('click', function () {
        var quantidade = document.id('quantidade').value;
        var de = document.id('de').value;
        var para = document.id('para').value;
        fazerRequest(quantidade, de, para);
    });
});
var fazerRequest = function (valor, origem, destino) {
    new Request.exchange({
        from: origem,
        to: destino,
        amount: valor,
        onSuccess: function (response) {
            document.id('resultado').set('html', response.v + ' ' + destino);
        }
    }).send();
}