That’s a very interesting point, thanks. I want to clarify though, since you say "call" and "reevaluate", that this not calling a function but accessing a property of the (immutable) string.

How this is handled internally by the JS engine is a different question; there’s a more in-depth discussion that touches on this on StackOverflow. In short, caching .length has no effect on real-world performance in V8, Firefox, etc.

(Note that in this case the entire issue can be avoided by using e.g. str.slice(0, -1).)