Why for loop with cached length is slower than simple for loop?

Question

I am reading the book "Secrets of the JavaScript Ninja". But I am confused with the figure that I found on in this book. From my view, simple for loop lost time to get the length of array so if the length is cached before, the speed would be improved. But the picture below showed the opposite result. Could someone give me the reason? .Thanks.

Answer 1

Performance in js is a question of how much the engine optimized the code. And how the engine optimizes code depends on the developers that wrote the optimization. And those developers want the average code to run fast, that means the more "normal" your code is, the faster it runs. So maybe iterating over an array in the first way is so common that someone heavily optimized it. Whatsoever the only real result we can gain from that data is: both ways are fast enough to not care about the difference.