To be fair the AST structure can also be implemented more efficiently without better control over memory layout. The JS ecosystem standardized on polymorphic AS

To be fair the AST structure can also be implemented more efficiently without better control over memory layout. The JS ecosystem standardized on polymorphic ASTs, which in retrospect seems dumb, but is not a result of any fundamental limitation in JS. E.g. in ESTree evaluating such a common expression as `node.type` is actually really expensive -- it incurs the costs of a hashmap lookup (more or less) where you'd expect it to be able to be implemented using pointer arithmetic.