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mathias's profile
Mathias Bynens
Mathias Bynens
Mathias Bynens
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@mathias

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Mathias BynensVerified account

@mathias

I work on @ChromeDevTools & @v8js at Google and on ECMAScript through TC39. ♥ JavaScript, HTML, CSS, HTTP, performance, security, Bash, Unicode, i18n, macOS.

Munich, Germany
mths.be
Joined January 2007

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    1. Jake Archibald‏ @jaffathecake 8 Feb 2018
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      @smashingconf question for @mathias: const arr = []; let i = length; while (i--) arr[i] = whatever; Does are therefore become a holy array and deopt?

      3 replies 0 retweets 8 likes
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    2. Jake Archibald‏ @jaffathecake 8 Feb 2018
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      Also, what about new Array(20).fill(0)? Packed or holy?

      4 replies 0 retweets 2 likes
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      Mathias Bynens‏Verified account @mathias 8 Feb 2018
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      Replying to @jaffathecake

      Both arr.length = n and new Array(n).fill(0) (for n > 0) create holey arrays. No way to transition upwards in the lattice, even with Array#fill 😥 (We could change this in @v8js, but it would be a massive undertaking that’s likely not worth the performance benefits.)

      3:16 AM - 8 Feb 2018
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      4 replies 0 retweets 5 likes
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        2. Ingvar Stepanyan‏ @RReverser 8 Feb 2018
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          Replying to @mathias @jaffathecake @v8js

          Why can't `fill` always transition to non-holey? Is there a case where it would still leave holes in array?

          1 reply 0 retweets 0 likes
        3. Jake Archibald‏ @jaffathecake 8 Feb 2018
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          Replying to @RReverser @mathias @v8js

          It's possible, but not currently seen as worth it. Typed arrays are usually a better answer.

          0 replies 0 retweets 0 likes
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        2. Ashley Claymore‏ @acutmore 8 Feb 2018
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          Replying to @mathias @jaffathecake @v8js

          Array.from({ length: n }, (_, i) => i) Holey? It’s my goto ‘range’ method

          1 reply 0 retweets 0 likes
        3. Mathias Bynens‏Verified account @mathias 8 Feb 2018
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          Replying to @acutmore @jaffathecake @v8js

          Yes, that array would be marked as holey. See https://mths.be/v8ek .

          0 replies 0 retweets 0 likes
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        2. Axel Rauschmayer‏ @rauschma 8 Feb 2018
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          Replying to @mathias @jaffathecake @v8js

          How about a new factory method Array.create(len, initValue)?

          1 reply 0 retweets 5 likes
        3. Jake Archibald‏ @jaffathecake 8 Feb 2018
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          Replying to @rauschma @mathias @v8js

          The feeling is this isn't worth it vs just use a typed array

          1 reply 0 retweets 1 like
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        2. Sasha Chu‏ @Chudesnov 8 Feb 2018
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          Replying to @mathias @jaffathecake @v8js

          And what if I eval an expression that uses only [] and object/primitive references?

          1 reply 0 retweets 0 likes
        3. Mathias Bynens‏Verified account @mathias 8 Feb 2018
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          Replying to @Chudesnov @jaffathecake @v8js

          The only way to create a hole within an array literal itself is to repeat a comma: [1, 2, , 3] If you don’t do that, the array remains packed. Also, have you considered using `JSON.parse` instead of `eval`?

          1 reply 0 retweets 1 like
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