Aight,

It seems its depends on my framework apparently. Im using knockout.js framework hence my declaration for a function is different that causes the response callback execution issues.

(normal javascript function calling)

function responseHandler(response) {
    if (response.messages.resultCode === 'Error') {
        for (var i = 0; i < response.messages.message.length; i++) {
            console.log(response.messages.message[i].code + ':' + response.messages.message[i].text);
        }
    } else {
        useOpaqueData(response.opaqueData)
    }
}

(knockoutjs function calling)

  responseHandler = function(response){
                     if (response.messages.resultCode === 'Error') {
                        for (var i = 0; i < response.messages.message.length; i++) {
                            console.log(response.messages.message[i].code + ':' + response.messages.message[i].text);
                        }
                    } else {
                        useOpaqueData(response.opaqueData)
                    }
                }

I'm replying this anyways since some people might think its the Accept.js issues, apparently its not. Check your framework documentation just to make sure how the function calling is called.