Checks the “Content-Type” header of the response and handles “application/json” responses specially. Another response type that you might want to “decode” specially is “application/javascript” or “text/javascript”.

ParsingHTTPResponse.js

// Issue an HTTP GET request for the contents of the specified URL.
// When the response arrives, pass it to the callback function as a 
// parsed XML Document object, a JSON-parsed object, or a string.
function get(url, callback) {
 var request = new XMLHttpRequest(); // Create new request
 request.open("GET", url); // Specify URL to fetch
 request.onreadystatechange = function() { // Define event listener
 // If the request is compete and was successful
 if (request.readyState === 4 && request.status === 200) {
 // Get the type of the response
 var type = request.getResponseHeader("Content-Type");
 // Check type so we don't get HTML documents in the future
 if (type.indexOf("xml") !== -1 && request.responseXML) 
 callback(request.responseXML); // Document response
 else if (type === "application/json")
 callback(JSON.parse(request.responseText)); // JSON response
 else 
 callback(request.responseText); // String response
 }
 };
 request.send(null); // Send the request now
}