If you’ve used Google Search recently, you may have noticed a new feature that we’re calling Instant Previews. By clicking on the (sprited) magnifying glass icon next to a search result you see a preview of that page, often with the relevant content highlighted. Once activated, you can mouse over the rest of the results and quickly (instantly!) see previews of those search results, too.
Although error handling with JSONP is a bit harder to do compared to XHR (not all browsers support onerror events), JSONP can be cached aggressively by the browser, and is not subject to same-origin restrictions. This last point is important for Instant Previews because web browsers restrict the number of concurrent requests that they send to any one host. Using a different host for the preview requests means that we don’t block other requests in the page.
There are a couple of tricks when using JSONP that are worth noting:
At this point you are probably curious as to what we’re returning in our JSONP calls, and in particular, why we are using JSON and not just plain images. Perhaps you even used Firebug or your browser’s Developer Tools to examine the Instant Previews requests. If so, you will have noticed that we send back the image data as sets of data URIs. Data URIs are base64 encodings of image data, that modern browsers (IE8+, Chrome, Safari, Firefox, Opera, etc) can use to display images, instead of loading them from a server as usual.
We use caching throughout our implementation, but it’s important to not forget about client-side caching as well. By using JSONP and data URIs, we limit the number of requests made, and also make sure that the browser will cache the data, so that if you refresh a page or redo a query, you should get the previews, well... instantly!