This is trivially testable in most browsers inspection tools.

The answer is it works fine. x.a === y.a === 5

This is because you are simply declaring the binding of y to the object bound to x constant. This does not impact your ability to rebind x or to alter the contents of the object, it simply prevents you from rebinding y.

    let x = {a:5}
    x = {}
    console.log(x.a) // undefined

---

    const y = {a:5}
    y = {} // Uncaught TypeError: Assignment to a constant variable.

---

    let x = {a:2}
    const y = x
    x.b = 12    
    x = {}
    x.b = 13
    console.log(y) // {a: 2, b:12}
    console.log(x) // {b: 13}

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`.