Duff’s device in JavaScript raced hand-written loops that copy one array into another. A reader asked the follow-up: how do they compare with Array#copyWithin, the built-in that copies a range inside a single array? That is a different workload, so it needs its own measurement. The short answer: on every Node and Deno we can install today, the built-in runs 47 to 84 times slower than the loop.

“Premature optimization is the root of all evil” — our field’s favorite half-sentence, quoted far more often than the sentence it was cut from. Usually it serves as permission: build it fast, profile later, fix a thing or two, done. Let’s put the sentence back together and ask what it licenses: when is optimization premature, and when is “premature” the excuse?

In 1983, Tom Duff needed to copy memory into an output register faster than his compiler could manage, and wrote the most famous abuse of switch in the history of C. I ported his device to JavaScript and raced it against the plainest possible loop — and the verdict changed with the engine, the engine’s version, and the CPU underneath.