HTTP by <script>: JSONP

For certain kinds of content, a <script> element can be used as a useful alternative to XMLHttpRequest. Simply set the src attribute of a <script> (and insert it into the document if it isn’t already there) and the browser will generate an HTTP request to download the URL you specify. <script> elements are useful Ajax transports for one primary reason: they are not subject to the same-origin policy, so you can use them to request data from servers other than your own.

The technique of using a <script> element as an Ajax transport has come to be known as JSONP: it works when the response body of the HTTP request is JSON-encoded. The “P” stands for “padding” or “prefix”—this will be explained in a moment.