Of course, but the language spec severely limit what optimizations can be done. For example, V8 will happily give you a proper array if certain criteria is met

Of course, but the language spec severely limit what optimizations can be done. For example, V8 will happily give you a proper array if certain criteria is met, but it must always be ready for you trying to index it with a string, which means that it must attempt to parse the string as a numeric value. This means deoptimization checks, or in the worst case, a non-optimized array.