Use CryptoJS for CB-ACCESS-SIGN GDAX API header

gdax.js

// This code does the equivalent of the node code provided in the GDAX API docs
var secret = 'PYPd1Hv4J6/7x...';

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

var body = JSON.stringify(""); // body goes here

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

// Base64 decode the alphanumeric secret string
var key = CryptoJS.enc.Base64.parse(secret);

// Use it as key for SHA 256 HMAC 
var signedMessage = CryptoJS.HmacSHA256(message, key);

// base 64 encode the digest output before sending in header
var output = signedMessage.toString(CryptoJS.enc.Base64);
return output;