Lately I’m seeing a lot of this baked-in assumption that ChatGPT writes good/idiomatic/performant code, as if the code it generates is ~The Answer~. I guess it’s the modern version of copy/pasting from Stack Overflow. The code I’ve had it write isn’t universally terrible but it almost always has some low-hanging performance fruit (in JS for instance, it seems to love spreading arrays). I’m not sure how much I trust it to write good benchmark code!
Adding to the others here, yeah - it has to loop over the whole array, and it allocates a new one. The allocation itself costs something and it also creates work for the garbage collector (freeing the old one). So it’s a lot of work compared to a .push()! Sometimes you do need the immutability aspect though, because a lot of things use referential equality for change detection (React is a big one).
Spreading allocates a new array. Most things you can do with spread you can do with normal functions that either just directly access an entry or mutate an array directly. [first, …rest] is nice, but will always be slower than direct access.
Spreading an array requires looping over all the array elements which is O(n) compared to something like Array.push() which is just 0(1), although spreading has other benefits like copying rather than mutating
I've been seeing a lot more "ChatGPT only produces junk code" comments (case in point, this entire thread), and that hasn't really been the case either.
For majority of those benchmarks it produced the right code from the first go. It struggled more with Java VirtualThreads - probably fewer programs in the training set. It also had a tendency to overcomplicate things (adding unncecessary code). So there were a few iterations needed plus some hand edits.
