Slides for my #jsconfau18 presentation on @v8js internals for JavaScript developers: https://slidr.io/mathiasbynens/v8-internals-for-javascript-developers …pic.twitter.com/elyFdi5NNt
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I work on @ChromeDevTools & @v8js at Google and on ECMAScript through TC39.
JavaScript, HTML, CSS, HTTP, performance, security, Bash, Unicode, i18n, macOS.
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Slides for my #jsconfau18 presentation on @v8js internals for JavaScript developers: https://slidr.io/mathiasbynens/v8-internals-for-javascript-developers …pic.twitter.com/elyFdi5NNt
x = new Array(2); x[0] = 1; x[1] = 2; It's common to pre-allocate arrays in other languages like C++, why is this not "idiomatic" in JS? Who decides what's idiomatic JavaScript?
It’s based on how the language works, i.e. how it’s spec’d. In JavaScript, holes can trigger expensive prototype chain lookups, which makes this a suboptimal pattern in JS.
Going holey is not the end of the world. We can still optimize holey array operations! But they’re inherently less efficient than packed arrays.
The spec doesn't explain the performance difference of operations of arrays constructed with x=new Array(1); x[0]=1; vs x=[1] That's not obvious (not idiomatic?) to the user. It's a V8 detail that array don't convert back.
Why not tracking the number of holes in a holey array and repack it if it reaches 0?
…but it would be a major undertaking that’s unlikely to be worth the effort, especially since (again) we can still optimize holey arrays! [2/2]
This is why I included the slides at the end. `new Array(n)` has its uses, and there’s nothing wrong with using it when it makes sense. Just try and avoid it when you don’t absolutely need to.
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