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What, specifically, does the asynchronous code accomplish or allow that the older code does not?

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The newer code uses the ability of modern browsers to execute Javascript aysnchronously from rendering the page (effectively, it is executed in another thread rather than the main page rendering thread).

This allows the code to be placed at the top of the page without impacting perceived page rendering performance.

This in turn means that partial page loads (e.g. someone hitting the "Stop" button, or navigating on to another page) are more likely to be captured by the script. With the old code, if the user navigates away before the JavaScript section is loaded and executed, it is possible for the page impression not to be recorded properly.

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In the case of partial page loads (i.e. someone hits "stop" or the connection is lost), does Analytics report a pageview, or is there some facility for reporting a partial page load? –  tnorthcutt Jul 13 '10 at 1:18
    
I don't know for sure, but I would imagine that it will be all-or-nothing. I think the asynchronous code just increases the chance that the page load will be logged. –  Mark Hatton Jul 13 '10 at 9:56

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