Choosing CORS over JSONP over Inline… and Lessons Learned Using CORS

I recently created a very simple client-only (no server-side code) that loads needed data dynamically. In order to access data from another storage location (in my case the data came from a Windows Azure Blob), the application needed to make a choice: how to load the data.

It really came down to three choices:

  1. Load the data synchronously as the page loaded using an Inline script tag
  2. Load the data asynchronously as part of initial page load using JSONP
  3. Load the data asynchronously as part of initial page load using CORS

All 3 options effectively work within the Same Origin Policy (SOP) sandbox security measures that browsers implement. If access is not coming from a browser (but from, say, curl or a server application), SOP has no effect. SOP is there to protect end users from web sites that might not behave themselves.

Option 1 would be to basically have a hardcoded script tag load the data. One disadvantage of this is put perfectly by Douglas Crockford: “A <script src="url"></script> will block the downloading of other page components until the script has been fetched, compiled, and executed.” This means that the page will block while the data is loaded, potentially making the initial load appear a bit more visually chaotic. Also, if this technique is the only mechanism for loading data, once the page is loaded, the data is never refreshed, a potentially severe limitation for some applications; in the very old days, the best we could do was periodically trigger a full-page refresh, but that’s not state-of-the-art in 2014.

Option 2 would be to load the data asynchronously using JSONP. This is a fine solution from a user experience point of view: the page structure is first loaded, then populated once the data arrives. The client invokes the request using the XMLHttpRequest object in JavaScript.

Option 3 would be to load the data asynchronously using CORS. This offers essentially the identical user experience as option 2 and also relies on the XMLHttpRequest object.

Options 1 and 2 require that the data be encapsulated in JavaScript code. For option 2 with JSONP the convention is a function (often named callback) that simply returns a JSON object. The client making the call will then need to execute the function to get at the data. Option 1 has slightly more flexibility and could be simply a data structure declared with a known name like var mapData = ... which the client can access directly.

Option 3 with CORS is able to return the data directly. In that regard it is a little tiny bit more efficient since no bubble-wrap is needed – and is a lot safer since you are not executing a JavaScript function returned returned by a potentially untrusted server.

JSONP is not based on any official standard, but is common practice. CORS is a standard that is supported in modern browsers and comes with granular access policies. As an example, CORS policies can be set to allow access from a whitelist of domains (such as paying customers), while disallowing from any other domain.

For all three options there needs to be coordination between the client and the server since they need to agree on how the data is packaged for transmission. For CORS, this also requires browser support (see chart below). All options require that JavaScript is enabled in the client browser.

Summarizing CORS, JSONP, Inline

The following summary compares key qualities.

Inline JavaScript JSONP CORS Comments
Synchronous or Async Synchronous Async Async
Granular domain-level security no no yes In any of the three, you could also implement an authorization scheme. This is above and beyond that.
Risk no yes no JSONP requires that you execute a JavaScript function to get at the data. Neither of the other two approaches require that. There’s an extra degree of caution needed for JSONP data sources outside of your control.
Efficiency on the wire close close most efficient Both Inline and JSONP both wrap your data in JavaScript constructs. These add a small amount of overhead. Depending on what you are doing, these could add up. But minor.
Browser support full full partial
Server support full full partial Servers need to support the CORS handshake with browsers to (a) deny disallowed domains, and (b) to give browsers the information they need to honor restrictions
Supported by a Standard no no yes
Is it the future no no yes Safer. Granular security. Standardized. Max efficiency.

Lessons Learned Using CORS

Yes, my simple one-page map app (described here) ended up using CORS. In large part since it is mature, and the browser support (see below) was sufficient.

Reloading Browser Pages: In debugging, CTRL-F5 is your friend in Chrome, Firefox, and IE if you want to clear the cache and reload the page you are on. I did this a lot as I was continually enabled and disabling CORS on the server to test out the effects.

Overriding CORS Logic in CHROME: It turns out that Chrome normally will honor all CORS settings. This is what most users will see. Let’s call this “civilian mode” for Chrome. But there’s also a developer mode – which you enable by running chrome with the chrome.exe –disable-web-security parameter. It was initially confused since it seemed Chrome’s CORS support didn’t work, but of course it did. This is one of the perils of living with a software nerd; my wife had used my computer and changed this a long time ago when she needed to build some CORS features, and I never knew until I ran into the perplexing issue.

Handling CORS Rejection: Your browser may not not let your JavaScript code know directly that a remote call was rejected due to a CORS policy. Some browsers silently map 404 to 0 if against a CORS-protected resource. You’ll see this mentioned in the code for httpGetString.js (if you look at my sample code).

Testing CORS from curl: Helped by a post on StackOverflow, I found it very handy to look at CORS headers from the command line. Note that you need to provide SOME origin in the request for it to be valid CORS, but here’s the command that worked for my cloud-host resource (you should also be able to run this same command):

curl -H “Origin: http://localhost” -H “Access-Control-Request-Method: GET” -H “Access-Control-Request-Headers: X-Requested-With” -X OPTIONS –verbose

Browser Support for CORS

To understand where CORS support stands with web browsers, this fantastic site offers a nice visual showing CORS support across today. A corresponding chart for JSONP is not needed since it works within long-standing capabilities.



My simple one-page map app is described here. That page includes a link to a running instance and its source code is easily viewed with View Source.


3 thoughts on “Choosing CORS over JSONP over Inline… and Lessons Learned Using CORS

  1. Pingback: Stupid Azure Trick #6 – A CORS Toggler Command-line Tool for Windows Azure Blobs | Coding Out Loud

  2. Pingback: Stupid Azure Trick #7 – Use Windows Azure’s Local Storage Emulator with Web Sites | Coding Out Loud

  3. René

    Excellent article, well explained and where the virtues of both systems are in sight. I really clarified the situation, since I only used jsonp and jquery. With this article I aclré my doubts sovran CORS and see clearly the advantages of either method. Thank You


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