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.

The last post argued that break, continue, and labeled jumps are structured goto — disciplined jumps that keep code linear. Two ordinary language features are jumps in disguise: yield, which suspends a function and later resumes it in the middle, and throw, which leaps across stack frames to a waiting handler.

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.

“Go To Statement Considered Harmful” is one of the most-quoted titles in programming, and almost nobody reads past it. The argument underneath is narrower than the slogan it became — and the reflex it bred, avoiding every jump, produces code worse than the goto it was meant to replace.