New ways I: fetch
TL;DR I’d recommend this way as long as you don’t have to send synchronous requests or support old browsers.
A long as your request is asynchronous you can use the Fetch API to send HTTP requests. The fetch API works with promises, which is a nice way to handle asynchronous workflows in JavaScript. With this approach you use fetch() to send a request and ResponseBody.json() to parse the response:
fetch(url)
.then(function(response) {
return response.json();
})
.then(function(jsonResponse) {
// do something with jsonResponse
});
Compatibility: The Fetch API is not supported by IE11 as well as Edge 12 & 13. However, there are polyfills.
New ways II: responseType
As Londeren has written in his answer, newer browsers allow you to use the responseType property to define the expected format of the response. The parsed response data can then be accessed via the response property:
var req = new XMLHttpRequest();
req.responseType="json";
req.open('GET', url, true);
req.onload = function() {
var jsonResponse = req.response;
// do something with jsonResponse
};
req.send(null);
Compatibility: responseType="json" is not supported by IE11.
The classic way
The standard XMLHttpRequest has no responseJSON property, just responseText and responseXML. As long as bitly really responds with some JSON to your request, responseText should contain the JSON code as text, so all you’ve got to do is to parse it with JSON.parse():
var req = new XMLHttpRequest();
req.overrideMimeType("application/json");
req.open('GET', url, true);
req.onload = function() {
var jsonResponse = JSON.parse(req.responseText);
// do something with jsonResponse
};
req.send(null);
Compatibility: This approach should work with any browser that supports XMLHttpRequest and JSON.
JSONHttpRequest
If you prefer to use responseJSON, but want a more lightweight solution than JQuery, you might want to check out my JSONHttpRequest. It works exactly like a normal XMLHttpRequest, but also provides the responseJSON property. All you have to change in your code would be the first line:
var req = new JSONHttpRequest();
JSONHttpRequest also provides functionality to easily send JavaScript objects as JSON. More details and the code can be found here: http://pixelsvsbytes.com/2011/12/teach-your-xmlhttprequest-some-json/.
Full disclosure: I’m the owner of Pixels|Bytes. I thought that my script was a good solution for the original question, but it is rather outdated today. I do not recommend to use it anymore.