What is JSONP, and why was it created?
I understand JSON, but not JSONP. Wikipedia's document on JSON is (was) the top search result for JSONP. It says this:
JSONP or "JSON with padding" is a JSON extension wherein a prefix is specified as an input argument of the call itself.
Huh? What call? That doesn't make any sense to me. JSON is a data format. There's no call.
The 2nd search result is from some guy named Remy, who writes this about JSONP:
JSONP is script tag injection, passing the response from the server in to a user specified function.
I can sort of understand that, but it's still not making any sense.
So what is JSONP? Why was it created (what problem does it solve)? And why would I use it?
Addendum: I've just created a new page for JSONP on Wikipedia; it now has a clear and thorough description of JSONP, based on jvenema's answer.
It's actually not too complicated...
Say you're on domain example.com, and you want to make a request to domain example.net. To do so, you need to cross domain boundaries, a no-no in most of browserland.
The one item that bypasses this limitation is <script> tags. When you use a script tag, the domain limitation is ignored, but under normal circumstances, you can't really do anything with the results, the script just gets evaluated.
Enter JSONP. When you make your request to a server that is JSONP enabled, you pass a special parameter that tells the server a little bit about your page. That way, the server is able to nicely wrap up its response in a way that your page can handle.
For example, say the server expects a parameter called callback to enable its JSONP capabilities. Then your request would look like:
http://www.example.net/sample.aspx?callback=mycallback
Without JSONP, this might return some basic JavaScript object, like so:
{ foo: 'bar' }
However, with JSONP, when the server receives the "callback" parameter, it wraps up the result a little differently, returning something like this:
mycallback({ foo: 'bar' });
As you can see, it will now invoke the method you specified. So, in your page, you define the callback function:
mycallback = function(data){
alert(data.foo);
};
And now, when the script is loaded, it'll be evaluated, and your function will be executed. Voila, cross-domain requests!
It's also worth noting the one major issue with JSONP: you lose a lot of control of the request. For example, there is no "nice" way to get proper failure codes back. As a result, you end up using timers to monitor the request, etc, which is always a bit suspect. The proposition for JSONRequest is a great solution to allowing cross domain scripting, maintaining security, and allowing proper control of the request.
These days (2015), CORS is the recommended approach vs. JSONRequest. JSONP is still useful for older browser support, but given the security implications, unless you have no choice CORS is the better choice.
JSONP is really a simple trick to overcome the XMLHttpRequest same domain policy. (As you know one cannot send AJAX (XMLHttpRequest) request to a different domain.)
So - instead of using XMLHttpRequest we have to use script HTML tags, the ones you usually use to load js files, in order for js to get data from another domain. Sounds weird?
Thing is - turns out script tags can be used in a fashion similar to XMLHttpRequest! Check this out:
script = document.createElement('script');
script.type = 'text/javascript';
script.src = 'http://www.someWebApiServer.com/some-data';
You will end up with a script segment that looks like this after it loads the data:
<script>
{['some string 1', 'some data', 'whatever data']}
</script>
However this is a bit inconvenient, because we have to fetch this array from script tag. So JSONP creators decided that this will work better(and it is):
script = document.createElement('script');
script.type = 'text/javascript';
script.src = 'http://www.someWebApiServer.com/some-data?callback=my_callback';
Notice the my_callback function over there? So - when JSONP server receives your request and finds callback parameter - instead of returning plain js array it'll return this:
my_callback({['some string 1', 'some data', 'whatever data']});
See where the profit is: now we get automatic callback (my_callback) that'll be triggered once we get the data.
That's all there is to know about JSONP: it's a callback and script tags.
NOTE: these are simple examples of JSONP usage, these are not production ready scripts.
Basic JavaScript example (simple Twitter feed using JSONP)
<html>
<head>
</head>
<body>
<div id = 'twitterFeed'></div>
<script>
function myCallback(dataWeGotViaJsonp){
var text = '';
var len = dataWeGotViaJsonp.length;
for(var i=0;i<len;i++){
twitterEntry = dataWeGotViaJsonp[i];
text += '<p><img src = "' + twitterEntry.user.profile_image_url_https +'"/>' + twitterEntry['text'] + '</p>'
}
document.getElementById('twitterFeed').innerHTML = text;
}
</script>
<script type="text/javascript" src="http://twitter.com/status/user_timeline/padraicb.json?count=10&callback=myCallback"></script>
</body>
</html>
Basic jQuery example (simple Twitter feed using JSONP)
<html>
<head>
<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.6.2/jquery.min.js"></script>
<script>
$(document).ready(function(){
$.ajax({
url: 'http://twitter.com/status/user_timeline/padraicb.json?count=10',
dataType: 'jsonp',
success: function(dataWeGotViaJsonp){
var text = '';
var len = dataWeGotViaJsonp.length;
for(var i=0;i<len;i++){
twitterEntry = dataWeGotViaJsonp[i];
text += '<p><img src = "' + twitterEntry.user.profile_image_url_https +'"/>' + twitterEntry['text'] + '</p>'
}
$('#twitterFeed').html(text);
}
});
})
</script>
</head>
<body>
<div id = 'twitterFeed'></div>
</body>
</html>
JSONP stands for JSON with Padding. (very poorly named technique as it really has nothing to do with what most people would think of as “padding”.)
JSONP works by constructing a “script” element (either in HTML markup or inserted into the DOM via JavaScript), which requests to a remote data service location. The response is a javascript loaded on to your browser with name of the pre-defined function along with parameter being passed that is tht JSON data being requested. When the script executes, the function is called along with JSON data, allowing the requesting page to receive and process the data.
For Further Reading Visit: https://blogs.sap.com/2013/07/15/secret-behind-jsonp/
client side snippet of code
<!DOCTYPE html>
<html lang="en">
<head>
<title>AvLabz - CORS : The Secrets Behind JSONP </title>
<meta charset="UTF-8" />
</head>
<body>
<input type="text" id="username" placeholder="Enter Your Name"/>
<button type="submit" onclick="sendRequest()"> Send Request to Server </button>
<script>
"use strict";
//Construct the script tag at Runtime
function requestServerCall(url) {
var head = document.head;
var script = document.createElement("script");
script.setAttribute("src", url);
head.appendChild(script);
head.removeChild(script);
}
//Predefined callback function
function jsonpCallback(data) {
alert(data.message); // Response data from the server
}
//Reference to the input field
var username = document.getElementById("username");
//Send Request to Server
function sendRequest() {
// Edit with your Web Service URL
requestServerCall("http://localhost/PHP_Series/CORS/myService.php?callback=jsonpCallback&message="+username.value+"");
}
</script>
</body>
</html>
Server side piece of PHP code
<?php
header("Content-Type: application/javascript");
$callback = $_GET["callback"];
$message = $_GET["message"]." you got a response from server yipeee!!!";
$jsonResponse = "{\"message\":\"" . $message . "\"}";
echo $callback . "(" . $jsonResponse . ")";
?>
Because you can ask the server to prepend a prefix to the returned JSON object. E.g
function_prefix(json_object);
in order for the browser to eval "inline" the JSON string as an expression. This trick makes it possible for the server to "inject" javascript code directly in the Client browser and this with bypassing the "same origin" restrictions.
In other words, you can achieve cross-domain data exchange.
Normally, XMLHttpRequest doesn't permit cross-domain data-exchange directly (one needs to go through a server in the same domain) whereas:
<script src="some_other_domain/some_data.js&prefix=function_prefix>` one can access data from a domain different than from the origin.
Also worth noting: even though the server should be considered as "trusted" before attempting that sort of "trick", the side-effects of possible change in object format etc. can be contained. If a function_prefix (i.e. a proper js function) is used to receive the JSON object, the said function can perform checks before accepting/further processing the returned data.