Sure, the tmp-var-approach could be slower in some weird edge cases but probably only slightly, while the array-swapping approach could be much slower if the JS

Sure, the tmp-var-approach could be slower in some weird edge cases but probably only slightly, while the array-swapping approach could be much slower if the JS-Engine doesn't implement this optimization at all or this code-pattern is run in interpreter or baseline compiler. Not arguing you should do such micro-optimizations, but generally writing "simpler" code should also allow less sophisticated compilers to generate fast code.