Using jsPerf correctly

Using jsPerf correctly@mathias – #jsconfeu 2011

@mathias

jsPerf

jsPerf is 426 days old.24,164 test cases → ~56.72 per day83,536 tests → ~3.46 per test case1,012 comments → ~2.38 per day

“68% of all jsPerf test cases is broken.” — Jean-Jacques Rousseau

“It would be unwise to takeall jsPerf results for granted.” — William Shakespeare

Benchmarking pitfalls “Bulletproof JavaScript benchmarks” — http://mths.be/asv

Benchmarking pitfalls• Inaccurate millisecond timers “Bulletproof JavaScript benchmarks” — http://mths.be/asv

Benchmarking pitfalls• Inaccurate millisecond timers• Browser bugs “Bulletproof JavaScript benchmarks” — http://mths.be/asv

Benchmarking pitfalls• Inaccurate millisecond timers• Browser bugs• No statistically signiﬁcant results “Bulletproof JavaScript benchmarks” — http://mths.be/asv

Benchmarking pitfalls• Inaccurate millisecond timers• Browser bugs• No statistically signiﬁcant results• Browser plugins and addons “Bulletproof JavaScript benchmarks” — http://mths.be/asv

Benchmarking pitfalls• Inaccurate millisecond timers• Browser bugs• No statistically signiﬁcant results• Browser plugins and addons• No cross-browser testing “Bulletproof JavaScript benchmarks” — http://mths.be/asv

Benchmarking pitfalls• Inaccurate millisecond timers• Browser bugs• No statistically signiﬁcant results• Browser plugins and addons• No cross-browser testing• Incorrect tests “Bulletproof JavaScript benchmarks” — http://mths.be/asv

Unfaircomparisons

Function declarationsgo in the preparationcode or in the setup.

Only the bare minimumgoes inside each testbody.

http://jsperf.com/string-reverse

http://jsperf.com/ﬁbonacci-numbers

Make sure the methodsyou’re comparingactually do the samething.

If not, mention this inthe description of thetest case.

Make sure you’re testingwhat you intended totest.

http://jsperf.com/jquery-id-vs- native-getelementbyid

When re-using variablesacross tests, make sureyou reset them asneeded.

http://jsperf.com/a-b-vs-a-a-b/2

Don’t introducerandomness in your tests.

http://jsperf.com/zeropad

Don’t test asynchronousstuff synchronously.

http://jsperf.com/settimeout-vs-setinterval/4

Asynchronous tests — a good example: http://jsperf.com/smallest-timeout

There’s a differencebetween preparation, setupand teardown.

Example: scope lookups http://jsperf.com/truth

Example: DOM cleanuphttp://jsperf.com/basic-dom-manipulation

jsPerf uses Benchmark.js,which allows for advancedusage. http://benchmarkjs.com/docs

http://jsperf.com/removechild

This messes with your results, too:

This messes with your results, too:• Unexpected browser features

This messes with your results, too:• Unexpected browser features• Dead code removal

This messes with your results, too:• Unexpected browser features• Dead code removal•…

Questions? @mathias ~ @jsprf

http://jsconf.eu	14522
http://paper.li	53
http://lanyrd.com	34
http://lanyrd.com	34
http://a0.twimg.com	20
http://a0.twimg.com	20
http://us-w1.rockmelt.com	15
http://translate.googleusercontent.com	6
http://tweetedtimes.com	5
http://tweetedtimes.com	5
http://blog.sinnerschrader.de	3
http://webcache.googleusercontent.com	2
http://webcache-exp-test.googleusercontent.com	2
http://lanyrd.dev	1
http://api.snipdocs.com	1

Using jsPerf correctly

by Mathias Bynens on Oct 02, 2011

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