September 2007
Intermediate to advanced
170 pages
4h 24m
English
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Duplicate Scripts Hurt Performance
There are two ways that duplicate scripts hurt performance: unnecessary HTTP requests and wasted JavaScript execution.
Unnecessary HTTP requests happen in Internet Explorer, but not in Firefox. In Internet Explorer, if an external script is included twice and is not cacheable, the browser generates two HTTP requests during page loading. This is demonstrated in the "Duplicate Scripts—Not Cached" example.
- Duplicate Scripts—Not Cached
This won't be an issue for people who follow the advice in Chapter 5 and add a far future
Expires header to their scripts, but
if they don't, and they make the mistake of including the script twice,
the user has to endure an extra HTTP request. Chapter 8 explains how
downloading scripts has an especially negative impact on response times.
Subjecting the user to an extra HTTP request for a script doubles that
negative impact.
Even if the script is cacheable, extra HTTP requests occur when the user reloads the page. The following example includes scripts that are cacheable.
- Duplicate Scripts—Cached
Load this page once to fill the cache, and then click the "Example 2 - Duplicate Scripts - Cached" link to load it again. Since the script is cached, no HTTP requests are made for the script, but if you click the browser's Refresh button, two HTTP requests are made. Specifically, two conditional GET requests are made. For more information, ...
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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.