I've been using Postman to test my REST API (written in Go). Now is the moment of truth, connecting to the front end and I've hit a hiccup. I'm using jQuery.ajax to send a GET to my server, which should then send me back the response.

Note: Going to postman and sending a GET request to http://REMOTE_IP:8000/users/ returns the information I'm looking for, an array of User objects (JSON). It confuses me that it would work for Postman, but not jQuery.

jQuery Get

$.ajax({
    url: "http://REMOTE_IP:8000/users/",
    type: "GET",
    dataType: 'jsonp',
    contentType: 'application/json',
    success: function(data) {
        console.log(data);
    },
    error: function(jqXHR, status, err) {
        console.log(jqXHR);
        console.log(status);
        console.log(err);
    }
});

Go Backend

func GetUsersRoute(w http.ResponseWriter, r *http.Request) {
    var queryResult []models.User

    queryErr := Config.Users.Find(bson.M{}).All(&queryResult)
    resultsJSON, jsonErr := json.Marshal(queryResult)

    handleQueryAndJsonErrors(w, queryErr, jsonErr)

    // added these for cors, not sure they do anything helpful
    w.Header().Set("Content-Type", "application/json; charset=UTF-8")
    w.Header().Set("Access-Control-Request-Headers", "*")

    w.WriteHeader(http.StatusOK)
    w.Write(resultsJSON)
}

I also have a logger attached to my go backend, and here are the request method and request URI that are printed out when I make the request:

GET /users?callback=jQuery223019655483671137586_1461946990647&_=1461946990648

Here are some articles I found useful:
The data which jquery $.ajax function return could not be parse to json

https://github.com/codegangsta/martini-contrib/blob/master/cors/cors.go

https://developer.mozilla.org/en-US/docs/Web/HTTP/Access_control_CORS#Access-Control-Allow-Methods

Here are the errors returned with jQuery's err callback:

What could be going wrong?

# Answer

I found a nice go library that handles the cross-origin problems I was having, meaning I didn't have to rely on jsonp. Here's the revised go code:

Using the CORS Handler

handler := cors.Default().Handler(r)
err := http.ListenAndServe(port, handler)

if err != nil {
    panic(err)
}

Revised handler

func GetUsersRoute(w http.ResponseWriter, r *http.Request) {
    var queryResult []models.User

    queryErr := Config.Users.Find(bson.M{}).All(&queryResult)
    resultsJSON, jsonErr := json.Marshal(queryResult)

    handleQueryAndJsonErrors(w, queryErr, jsonErr)

    w.Header().Set("Content-Type", "application/json; charset=UTF-8")
    w.WriteHeader(http.StatusOK)
    w.Write(resultsJSON)
}

Calling from the frontend

var extension = 'users/',
    requestPath = serverAddress + extension;

$.ajax({
    url: requestPath,
    type: 'GET',
    dataType: 'json',
    contentType: 'application/json',
    success: successCall,
    error: failCall
});

First check whether your server can handle JSONP requests by using following tests.

http://REMOTE_IP:8000/users?callback=myFunction

The response instead of this (it will look like this if it doesn't support JSONP):

{ "thing": "value" .... }

Should look like this (again, if it supports JSONP):

myFunction({ "thing": "value" .... });