https://github.com/sliptree/money.js

http://openexchangerates.github.io/money.js/

<script src="path/to/money.js"></script>

// Using defaults:
fx.settings = { from: "USD", to: "GBP" };
fx.convert(1000); // 647.71034

// With options:
fx.convert(1000, {to: "HKD"}); // 7781.91949 (default `from` currency)
fx.convert(1000, {from: "GBP", to: "HKD"}); // 12014.50549