It’s O(1), like the equivalent structure in other languages, but individual timings are influenced by other factors, like cache, as seen here. Still with a constant bound for all intents and purposes.
> Another interesting point to note here is that in the second half of the article, the author modified the `localAccess` function to accept an optional array. That may actually have side effects that are unaccounted for, which is clear upon comparison of Entry 1 and Entry 3 for all environments; and Entry 2 and Entry 4 for only Firefox. If we ignore the order-of-magnitude difference and focus on the trends, it appears that the optimisation used in different engines seen here corroborates what @londons_explore has already pointed out.
The margin of error is huge for 1, and 1 and 3 are within margins of error for the V8s, so my guess is that you ran them in the same order every time and the difference is just an artefact of being the first to run. You should wait for the page to settle down to start tests and try reordering or separating them.