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

Yes, this is expected behaviour.

it fails silently.

Not exactly. Or: Only in sloppy mode. If you "use strict" mode, you'll get an

Error { message: "Invalid assignment in strict mode", … }

on the line test4.answer = function() { return 0; };

it can be overloaded on the prototype of a subclass (test2), but not an instance of a subclass (test4)

This has nothing to do with instances vs. prototypes. What you didn't notice is that you're using different ways to create the overloading property:

• an assignment to a property that is inherited and non-writable fails
• an Object.defineProperty call just creates a new property, unless the object is non-extensible

You can do the same for your instance:

Object.defineProperty(test4, "answer", {
    value: function() { return 42; }
});