[pfooti]

I'm torn here. I agree that this is one of the Nice Things about javascript - I can do stuff like

  let foo = opts.foo || 'default'

Which is nice. On the other hand, if you look at how V8 does its JIT compiling, it seems like there's just some things you can't optimize around, and they've gotten as far as they can reasonably be expected to get there. Having object schema that can change on the fly is just really hard to JIT efficiently.

My worry is that Strong Mode becomes the defacto standard, and we end up losing the flexibility and expressiveness of well-written JS, and end up with static typing all over. If your JS is compact and otherwise well-formed, you can probably afford the compiler hit sometimes, knowing that code is a lot easier to write.