Does Javascript writable descriptor prevent changes on instances?

Question

Answers (please read them below, their respective authors provided valuable insights):

• \"writable: false\" prevents assigning a new value,

Answer 1

You cannot write to an instance property if its prototype defines that property as unwritable (and the object instance doesn't have a descriptor) because the set operation goes up to the parent (prototype) to check if it can write, even though it would write to the child (instance). See EcmaScript-262 Section 9.1.9 1.c.i

4. If ownDesc is undefined, then
  a. Let parent be O.[[GetPrototypeOf]]().
  b. ReturnIfAbrupt(parent).
  c. If parent is not null, then
      i. Return parent.[[Set]](P, V, Receiver).

However, if you are trying to get around that, you can set the descriptor of the instance itself.

var proto = Object.defineProperties({}, {
  foo: {
    value: "a",
    writable: false,  // read-only
    configurable: true  // explained later
  }
});

var instance = Object.create(proto);
Object.defineProperty(instance, "foo", {writable: true});
instance.foo // "b"