JSConf 2015 John David Dalton
summary
Browsers implement many built-ins as built-ins implemented in JavaScript, which lets engines like chakra and ms edge ship and iterate fast and lets contributors read and tweak code such as the Intl implementation and array indexOf shimlike code, but that choice forces costly guards against tainting and stack leaks and hides engine internals so heuristics hurt performance and cross-engine results vary wildly — for example Object.assign is ~62% slower on v8 in some tests while chakra is much faster.
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