The advantage of lexical scope over dynamic scope is predictability. You can reliably* read a javascript program and map each variable access to the scope of th

The advantage of lexical scope over dynamic scope is predictability. You can reliably* read a javascript program and map each variable access to the scope of that variable (so can a compiler, which accounts for no small portion of the speed of modern javascript implementations). With dynamic scoping there are no guarantees about where you'll find any nonlocal variable. In practice this caused enough difficulty that the programming language community has moved steadily away from dynamic scoping. You make an interesting parallel with prototype lookup, but it's worth noting that the prevailing pattern of prototype use mimics the traditional class and instance rubric. In that situation it is also fairly easy and deterministic to pin down where in the prototype chain every identifier is defined. * with the exception of code which uses `with` statements and certain forms of eval, which is a big part of why those constructs are strongly discouraged by the community