This document discusses best practices for using jsPerf, a tool for benchmarking JavaScript performance. It outlines common pitfalls to avoid like inaccurate timers, browser bugs, lack of statistical significance, unfair comparisons, asynchronous tests, and not properly setting up, preparing, and tearing down tests. The key recommendations are to only include the minimum code in each test, reset variables between tests, avoid randomness, don't test asynchronous code synchronously, and understand how Benchmark.js works for advanced usage. Following these guidelines will produce more accurate and reliable benchmark results.

1. Using jsPerf correctly
@mathias – #jsconfeu 2011

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3. @mathias

4. jsPerf

10. jsPerf is 426 days old.
24,164 test cases → ~56.72 per day
83,536 tests → ~3.46 per test case
1,012 comments → ~2.38 per day

11. jsPerf is 426 days old.
24,164 test cases → ~56.72 per day
83,536 tests → ~3.46 per test case
1,012 comments → ~2.38 per day

12. “68% of all jsPerf test cases
is broken.”
— Jean-Jacques Rousseau

13. “It would be unwise to take
all jsPerf results for granted.”
— William Shakespeare

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

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

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

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

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

19. 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

20. 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

21. 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

22. 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

23. Unfair
comparisons

24. Function declarations
go in the preparation
code or in the setup.

26. Only the bare minimum
goes inside each test
body.

28. http://jsperf.com/string-reverse

31. http://jsperf.com/fibonacci-numbers

33. Make sure the methods
you’re comparing
actually do the same
thing.

34. If not, mention this in
the description of the
test case.

36. Make sure you’re testing
what you intended to
test.

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

39. When re-using variables
across tests, make sure
you reset them as
needed.

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

43. Don’t introduce
randomness in your tests.

44. http://jsperf.com/zeropad

45. Don’t test asynchronous
stuff synchronously.

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

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

50. There’s a difference
between preparation, setup
and teardown.

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

57. Example: DOM cleanup
http://jsperf.com/basic-dom-manipulation

61. jsPerf uses Benchmark.js,
which allows for advanced
usage.
http://benchmarkjs.com/docs

62. http://jsperf.com/removechild

64. This messes with your results, too:

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

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

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

68. Questions?
@mathias ~ @jsprf