V8 對 for-in 的最佳化

V8 引擎的人對 for-in 的最佳化寫了一篇解釋「Fast For-In in V8」,比較直接的結果就是維基百科Facebook 都變快了:

For example, in early 2016 Facebook spent roughly 7% of its total JavaScript time during startup in the implementation of for-in itself. On Wikipedia this number was even higher at around 8%.

可以看得出來是挑比較大的來改,而下一版的 Google Chrome (57) 將會對 for-in 會到另外一個極致:

The most important for-in helpers are at position 5 and 17, accounting for an average of 0.7% percent of the total time spent in scripting on a website. In Chrome 57 ForInEnumerate has dropped to 0.2% of the total time and ForInFilter is below the measuring threshold due to a fast path written in assembler.

主要是因為 spec 對 for-in 的定義寫得很模糊,所以就有很多實作的空間可以調整:

When we look at the spec-text of for-in, it’s written in an unexpectedly fuzzy way,which is observable across different implementations.

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