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與 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.