What are the benefits and drawbacks of loading JavaScript asynchronously?

You navigate to a web page, expecting to see today’s special offers from your favourite retailer. Instead, you see a blank screen. You wait. And wait. But nothing happens. Eventually, you give up and decide to leave. It turns out that they had a synchronous script in their document head, which timed out, stopping the page in its tracks.

This is why we quite often talk about how useful it is to load JavaScript asynchronously. But what does this actually mean? Is it the best thing to do in every case? And are there alternatives?
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The latest addition to our new monitoring portal – Google Analytics overlay

The new user interface for our monitoring service is rapidly heading towards completion. We’re nearing feature parity with the existing interface, and we’re adding plenty of new things too.

To access the new version, simply click on the test tube icon next to your monitoring account name (under “Portal v8 Beta”):

test-tube

In the latest release, we’ve made it possible for you to overlay Google Analytics data on three reports: Speed vs Time (Component), Speed vs Time (Severity) and Speed vs Time Object (Component).
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