Coinbase Exchange signing with Node.js

gistfile1.js

var crypto = require('crypto');

var secret = 'PYPd1Hv4J6/7x...';

var timestamp = Date.now() / 1000;
var req_url = '/orders';

var body = JSON.stringify({
    price: '1.0',
    size: '1.0',
    side: 'buy',
    product_id: 'BTC-USD'
});

var method = 'POST';

// create the prehash string by concatenating required parts
var what = timestamp + method + req_url + body;

// decode the base64 secret
var key = Buffer(secret, 'base64');

// create a sha256 hmac with the secret
var hmac = crypto.createHmac('sha256', key);

// sign the require message with the hmac
// and finally base64 encode the result
return hmac.update(what).digest('base64');