CORS

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CORS

CORS

Cross-Origin Resource Sharing (CORS) is a mechanism for requesting fonts, scripts, and other resources from an origin (defined, as above, as the combination of domain, protocol, and port) other than the requesting origin. For instance, if sending a request from http://www.example.com, any of the following would be "cross origin":

  • http://mail.example.com (domain differs),
  • https://www.example.com (protocol differs),
  • http://www.example.com:8080 (port differs),

and, thus, scripts (or font, or other similar resources) would be blocked from these "foreign" sites. CORS offers a way for two sites to allow safe sharing of resources.

How CORS Works

CORS defines the communication between browser and server: specific headers in the HTTP request and HTTP response tell the browser that its OK to accept the resource. At its most basic, a server issuing an HTTP response which includes the header

Access-Control-Allow-Origin: *

is allowing access from all requesting domains. A more-restrictive response, for example

Access-Control-Allow-Origin: http://www.example.com

would allow access only from a particular domain.

The great thing for us, as web developers, is that CORS-enabled responses work just like responses from our own (same-origin) site: our code can process the JSON, XML, or other response we receive just as if we were making a request of a page or resource on our own server.

The CORS request/response cycle can get significantly more complex, with "preflight" requests sent by the browser and responded to from the server, before another set of request/response; the passing of cookies or other authentication mechanisms; and other sharing of data. Check out http://www.html5rocks.com/en/tutorials/cors/, an excellent tutorial on HTML5 Rocks, to delve deeper into the topic.

Check out http://enable-cors.org/resources.html#apis for a list of sites that offer CORS-enabled resources.

Let's look at an example, accessing a CORS-enabled site and a not-CORS-enabled site. Open CORSJSONP/Demos/cors-html5-rocks.html in your browser and in a code editor to review the code. No need to start the Node.js server for this example.

Code Sample:

CORSJSONP/Demos/cors-html5-rocks.html
<!DOCTYPE HTML>
<html>
<head>
---- C O D E   O M I T T E D ----

<script type="text/javascript" src="jquery-1.11.1.min.js"></script>
<script type="text/javascript">
	$(document).ready(function() {
		$('#btnhtml5rocks').click(function() {
			$('#ResponseContent h2').html('Response Content');
			$('#ResponseContent div').html('');
			$.ajax({
				type: 'GET',
				url: 'http://www.html5rocks.com/en/tutorials/file/xhr2/',
				success: function(response) {
					var article = $(response).find('article').first().html();
					$('#ResponseContent h2').html('Response Content - from HTML5Rocks');
					$('#ResponseContent div').html(article);
				}
			}).fail(function() {
				alert("failed!");
			});;
		});
		$('#btnnytimes').click(function() {
			$('#ResponseContent h2').html('Response Content');
			$('#ResponseContent div').html('');
			$.ajax({
				type: 'GET',
				url: 'http://www.nytimes.com/',
				success: function(response) {
					var article = $(response).find('article').first().html();
					$('#ResponseContent h2').html('Response Content - from NY Times');
					$('#ResponseContent div').html(article);
				}
			}).fail(function() {
				alert("failed!");
			});
		});
	});
</script>
</head>
<body>
	<button id="btnhtml5rocks">Fetch HTML5 Rocks</button>
	<button id="btnnytimes">Fetch NY Times</button>
	<br>
	<div id="ResponseContent">
		<h2>Response Content</h2>
		<div></div>
	</div>
</body>
</html>

We include jQuery via a script tag, and use jQuery to effect our Ajax calls.

The page presents two buttons, with ids btnhtml5rocks and btnnytimes, respectively, and use jQuery to listen for a click on each button.

When either button is clicked, we make an Ajax call to a remote site, setting the h2 tag's content to show from which source we receive the external data and setting the contents of #ResponseContent div, if successful in our Ajax call, to the contents of the first article tag from our received response.

Clicking the "Fetch HTML5 Rocks" button generates an Ajax call (via jQuery's $.ajax method) to http://www.html5rocks.com/en/tutorials/file/xhr2/. We set local JavaScript variable article to the contents of the first article found in the returned response and display the contents on our page.

Despite our making a call to a cross-origin (i.e. non-local) site, our code works. Specifically, it is the presence of the Access-Control-Allow-Origin: * response header that tells our browser it is OK to allow this Ajax call:

HTML5 Rocks

Clicking the "Fetch NY Times" button, conversely, doesn't work: the jQuery method .fail(), which we chain on to the end of the $.ajax call to http://www.nytimes.com/, generates a popup alert. If we inspect the response headers, we would find no Access-Control-Allow-Origin: * among them. If we check the console, we find that our browser complains of our attempt to violate the same origin policy:

NY Times

In large part, CORS depends on the server responding with the appropriate headers; if that is the case then, conveniently, our work as client-side developers becomes relatively easy, pretty much the same as if we were working with resources on our own server.

Let's have you try out a call to a remote data source that sends back CORS-enabled headers.

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