The question was rhetorical to try to demonstrate that it might result in surprising behavior if const just manipulated its values to make them become immutable. If adding the `const y = x;` line made the object referenced in `x` be immutable, then the 3rd line would fail, which I don't is a consequence desired even by people who thought const made things immutable.

I guess one alternative idea for how const would work could be an implementation where `x.a = 5` worked but `y.a = 5` failed. But then what happens if you pass `y` into a function which then tries to assign the `a` property on it? Is the const-ness part of the value passed into the function, or is it a property on variable bindings, and you could only pass `y` to functions that accepted a const variable? That kind of function type checking isn't something usual to javascript currently. And then is the const-ness deep? Is `y.foo.a = 5;` blocked? Mutable DOM elements are a big part of javascript. If the object happened to contain a reference to an element that needed to be manipulated, then you won't be able to do something like `y.foo.element.textContent = "new foo content";`. Going down this road it's now getting to be a pretty big feature that doesn't cleanly fit with the rest of the language or common tasks.

Maybe the naming is a little unfortunate: Javascript's `const` has more in common with Java's `final` than C's `const`.