See examples for GLib libsoup at http://bobthegnome.blogspot.de/2015/11/accessing-json-webservice-using-libsoup.html

example-json.js

const Soup = imports.gi.Soup;
const Json = imports.gi.Json;

function assert(assertion) {
    if (!assertion) {
        throw "Error";
    }
}

/* Get the data using a HTTP GET */
let session = Soup.Session.new();
session.set_property(Soup.SESSION_USER_AGENT, "test-json");
let message = Soup.Message.new('GET', "https://reviews.ubuntu.com/reviews/api/1.0/review-stats/any/any");
assert(message != null)
let status_code = session.send_message(message);
assert(status_code == Soup.Status.OK)

/* Parse the data in JSON format */
let parser = Json.Parser.new();
let result = parser.load_from_data(message.response_body.data, -1);
assert(result);

/* The data should contain an array of JSON objects */
let _root = parser.get_root();
assert(_root.get_node_type() == Json.NodeType.ARRAY);
let array = _root.get_array(_root);
for (let i = 0; i < array.get_length(); i++) {
    /* Get the nth object, skipping unexpected elements */
    let node = array.get_element(i);
    if (node.get_node_type() != Json.NodeType.OBJECT) {
        continue;
    }

    /* Get the package name and number of ratings from the object - skip if has no name */
    let object = node.get_object();
    let package_name = object.get_string_member("package_name");
    let ratings_total = object.get_int_member("ratings_total");
    if (package_name) {
        print(package_name + ": " + ratings_total);
    }
}