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