September 2007
Intermediate to advanced
170 pages
4h 24m
English
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Chapter 8. Rule 6: Put Scripts at the Bottom
Chapter 7 described
how stylesheets near the bottom of the page prohibit progressive
rendering, and how moving them to the document HEAD eliminates the problem. Scripts (external
JavaScript files) pose a similar problem, but the solution is just the
opposite: it's better to move scripts from the top of the page to the
bottom (when possible). This enables progressive rendering and achieves
greater download parallelization. Let's first look at an example of these
problems.
Problems with Scripts
The best way to demonstrate the issues with scripts is by using an example that has a script in the middle of the page.
- Scripts in the Middle
This script is programmed to take 10 seconds to load, so it's easy to see the problem—the bottom half of the page takes about 10 seconds to appear (see the section "sleep.cgi" in Chapter 7 for an explanation of how components are configured to have specific load times). This occurs because the script blocks parallel downloading. We'll come back to this problem after a review of how browsers download in parallel.
The other problem with the example page has to do with progressive
rendering. When using stylesheets, progressive rendering is blocked
until all stylesheets have been downloaded. That's why it's best to move
stylesheets to the document HEAD, so
they are downloaded first and rendering isn't blocked. With scripts,
progressive rendering is blocked for all content
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O’Reilly covers everything we've got, with content to help us build a world-class technology community, upgrade the capabilities and competencies of our teams, and improve overall team performance as well as their engagement.Julian F.
I wanted to learn C and C++, but it didn't click for me until I picked up an O'Reilly book. When I went on the O’Reilly platform, I was astonished to find all the books there, plus live events and sandboxes so you could play around with the technology.Addison B.
I’ve been on the O’Reilly platform for more than eight years. I use a couple of learning platforms, but I'm on O'Reilly more than anybody else. When you're there, you start learning. I'm never disappointed.Amir M.
I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.