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Editorial Team
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Asked: 2026-05-23T21:25:33+00:00

I just ran into a very interesting issue when someone posted a jsperf benchmark

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I just ran into a very interesting issue when someone posted a jsperf benchmark that conflicted with a previous, nearly identical, benchmark I ran.

Chrome does something drastically different between these two lines:

new Array(99999);  // jsperf ~50,000 ops/sec
new Array(100000); // jsperf ~1,700,000 ops/sec

benchmarks: http://jsperf.com/newarrayassign/2

I was wondering if anyone has any clue as to what’s going on here!

(To clarify, I’m looking for some low-level details on the V8 internals, such as it’s using a different data structure with one vs the other and what those structures are)

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  1. Editorial Team

    Just because this sounded pretty interesting, I searched through the V8 codebase for a static defined as 100000, and I found this kInitialMaxFastElementArray var, which is the subsequently used in the builtin ArrayConstructInitializeElements function function. While I’m not a c programmer and don’t know the nitty-gritty here, you can see that it’s using an if loop to determine if it’s smaller than 100,000, and returning at different points based on that.

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