Is there anything that guarantees constant time for accessing a property of an object in JavaScript?

Is there anything in the JavaScript guaranteeing that the values are looked up in O(1) time?

No. JavaScript does not give any complexity guarantees whatsoever, except for ES6 collections.

I know the access operator [] gives a seasoned programmer the impression that he’s dealing with an O(1) lookup structure

Yes you are, this is a reasonable expectation. Engines employ all kinds of optimisations, from hidden classes over hashmaps to dynamic arrays, to meet these assumptions.

Of course, never forget that JS objects are complex beasts, and accessing a simple property might trigger a getter trap that in turn could do anything.

Can’t it be possible for a JavaScript engine to implement Object in a way such that properties are not looked up in O(1)?

Yes, that’s possible.

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