[esrauch]

By going through the dart compiler first they can do optimizations that you wouldn't do by hand. Google runs all their js through their Closure compiler for this (JS -> JS compiler) with a ton of optimizations turned on (constant folding, method inlining, deadcode elimination, replacing of multiline conditionals with ternary operator, etc, some of which I believe are in the open source version and possibly some proprietary).

In that case, they literally are writing JS and using a compiler to get JS that is faster than any normal handwritten JS would be. You could write your JS that way but it would be completely unreadable and unmaintainable.

[WhitneyLand]

Thank you for the clarification. I think we are saying the same thing.

In principle, it's always a certainty that JS can be written as performant as Dart, never just a possibility. The possibility part only comes in with respect how optimally the JS code is written.

The interesting part here is I think is how often does this happen? How often can significant improvements be made that would be impractical, or even unlikely for a developer to match?

[esrauch]

I suspect that depends on your definition of significant. I suspect any legible code can always get a few percent speedup by doing things that would make it unreadable and unmaintainable.