Yeah, that's fair, I've been bitten by "not actually immutable" bugs before (works well enough for primitives, but not objects or arrays, which is most things in JS-land!). Libraries like Immutable.js can help bridge the gap.

> they try to solve a bad design (`var`) but they introduce another bad design (`const`)

`let` is intended to be the actual `var` replacement, and it's okay for that. I think there's an argument that `const` is a bad design, in the sense that it's deceptive, under the not-that-uncommon case you describe. Still, I'd rather have it than not, and I personally still like defaulting to `const` and opting-in to `let` in the (rarer and rarer) event that it's needed. (I'm also on 100% TypeScript at this point, which doesn't remove the potential for problems, but does provide an extra layer of guard rails in general.)