nonconfigurable doesn't usually work as one might expect due to bug in JS spec

[jayphelps]

Guessing this is due to an apparent oversight in the spec, since every browser I tried had this behavior.

In raw JS, to show the issue: REPL

function Foo() {}

Object.defineProperty(Foo.prototype, 'bar', {
  value: 1,
  configurable: false,
  writable: true
});

Object.defineProperty(Foo.prototype, 'bar', {
  value: 2
});
// You might expect this to error but it doesn't!

Instead, it only does what you would expect if the descriptor is writable: false or is a getter/setter combo (which doesn't have a writable prop in the descriptor) REPL

function Foo() {}

Object.defineProperty(Foo.prototype, 'bar', {
  value: 1,
  configurable: false,
  writable: false
});

Object.defineProperty(Foo.prototype, 'bar', {
  value: 2
});
// TypeError: Cannot redefine property: bar

However this also obviously makes the property readonly, which isn't always desired and not the same thing as being nonconfigurable.

var foo = new Foo();
foo.bar = 3;
// TypeError: Cannot assign to read only property 'bar' of object '#<Foo>'

Most examples I've seen demoing configurable: false omit the writable property all-together, which means it defaults to writable: false so I imagine most haven't noticed this quirk.