Does the browser "cache" javascript code?

Question

I was fine tuning a page that is heavy on jquery and stumbled across this website:

http://www.componenthouse.com/extra/jquery-analysis.html

When I click on the "Run Test" button the first time, the numbers are way higher than subsequent clicks. Is this because JS is cached by the browser? Can someone explain how this works internally? Can a user choose to not cache the JS?

All my tests in 5 browsers returned stable values. I beleave your first test was slower because you were loading something else at the same time during this first test. — Havenard
– Havenard, Commented Sep 4, 2009 at 16:22

Paul · Accepted Answer · 2012-05-18 06:04:44Z

3

External javascript files are cached and, of course, an html containing script tags can be cached too. What you see may be a result of html caching or some browser optimization. You should try different browsers, closing and re-opening your browser and clearing the cache of the browser.

edited May 18, 2012 at 6:04

Paul

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answered Sep 4, 2009 at 16:18

Procrastinator2

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3 Comments

Alan Over a year ago

If you hit reload, you get slower times again, so it's not due to files being cached.

Eli Grey Over a year ago

What he's seeing is jQuery caching the selector query results.

Craig Stuntz Over a year ago

Elijah, I'm not sure that's true. Run it in the Firebug profiler and compare the results; the jQuery execution code times are similar on 1st and 2nd runs.

Craig Stuntz · Accepted Answer · 2009-09-04 16:20:03Z

1

The numbers are (significantly) different for me on the second time in Firefox 3.5. OTOH, they are fairly consistent(ly slow) in IE 8. Firefox 3.5's JavaScript interpreter compiles the JS to executable code. So it does make sense that the first time is slower; the code hasn't been JITted yet.

answered Sep 4, 2009 at 16:20

Craig Stuntz

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Comments

Duncan Beevers · Accepted Answer · 2009-09-04 16:21:15Z

1

The performance boost you're seeing is likely due to your javascript interpreter. Most newer web browsers use a JIT-compiling javascript engine so code paths taken multiple times can be optimized.

Read this blog post on how Safari's javascript engine achieved many of its speed-ups.

answered Sep 4, 2009 at 16:21

Duncan Beevers

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Eli Grey · Accepted Answer · 2009-09-04 16:32:52Z

1

Whether or not JavaScript code is cached, execution performance isn't affected. What you are seeing is jQuery caching the results for the selector queries so they don't take as long on subsequent runs.

edited Sep 4, 2009 at 16:32

answered Sep 4, 2009 at 16:18

Eli Grey

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4 Comments

Craig Stuntz Over a year ago

If that were the principal cause of the difference I'd expect to see similar behavior in IE 8.

Havenard Over a year ago

There's no performance gain in subsequent runs. Tested in 5 different browsers (and 2 different versions of IE), the number was stable from the first and all subsequent runs. The guy was probably loading much CPU during the first test and thought it had something to do with cacheing.

Craig Stuntz Over a year ago

Havenard, you and I are seeing different results, then. I see consistently faster execution on the second and all subsequent runs on Firefox 3.5.

Craig Stuntz Over a year ago

I'm seeing the same behavior (consistently faster on second and subsequent runs) in Google Chrome, too (though less dramatically faster; Chrome is faster to begin with).

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