JavaScript String split: fixed-width vs. delimited performance

Question

I am building a string to be parsed into an array by JavaScript. I can make it delimited or I can make the fields fixed-width.

I have only tested on Windows with Firefox and Chrome, so please run the test from other OSes and browsers. My two test results are clear: String.prototype.split() is the winner by a large margin.

Is my fixed-width code not efficient enough, or is the built-in string split function simply superior? Is there a way to code it so that the fixed-width parsing triumphs? If this was C/C++, the fixed-width code, written properly, would be the clear winner. But I know JavaScript is an entirely different beast.

I'd suggest putting your code inside a function to better hint the optimiser. But it's still just a microbenchmark. — Bergi
– Bergi, Commented Dec 28, 2019 at 15:21
@Bergi - so you're saying the question has no real answer unless I test it within the context of my real-world code in a single run with the largest plausible dataset? The total real-world performance difference might truly be negligible, but I wanted to see. This jsperf uses a small, but real-world dataset, so it seemed like a plausibly useful test. I just put stuff inside functions and the results are essentially the same. — Sideways S
– Sideways S, Commented Dec 28, 2019 at 15:26
I was thinking of something like this, which also fixes the bug where i < data.length should have been i < array.length - this increases perf by a factor of 16 :-) Still a huge difference though. — Bergi
– Bergi, Commented Dec 28, 2019 at 15:41
@Bergi - in Firefox your bug fix makes fixed(data) the winner, so kudos for that. But Firefox is so much slower than Chrome overall on this. Here is another sub-split variation that also shows slice() to be faster than split(). This one is not so much a microbenchmark because I really do loop over the array of strings and split each one: jsperf.com/ss-split-vs-slice — Sideways S
– Sideways S, Commented Dec 28, 2019 at 17:55

rustyx · Accepted Answer · 2019-12-28 16:10:21Z

2

String.prototype.split() is a built-in JavaScript function. Expect it to be highly optimized for the particular JS engine and be written not in JavaScript but in C++.

It should thus not come a surprise that you can't match its performance with pure JavaScript code.

String operations like splitting a delimited string are inherently memory-bound. Hence, knowing the location of delimiters doesn't really help much, since the entire string still needs to be traversed at least once (to copy the delimited fragments). Fixed-position splitting might be faster for strings that exceed D-cache size, but your string is just 13KB long, so traversing it multiple times isn't going to matter.

answered Dec 28, 2019 at 16:10

rustyx

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2 Comments

Bergi Over a year ago

"the entire string still needs to be traversed at least once (to copy the delimited fragments)" - that depends on the string representation, which might only create string objects referencing the same underlying buffer, with different offsets.

Sideways S Over a year ago

Thanks for the answer. I wasn't necessarily surprised, but I wanted to know. If you look at the comments to my original question you'll see some variation in this. In Firefox on Windows the revised fixed-width javascript code runs faster, but way slower than in Chrome.

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