Object Descriptors & Property Flags

Why does the last defineProperty call cause an error?

const person = {};

Object.defineProperty(person, 'age', {
  value: 25,
  writable: true,
  enumerable: true,
  configurable: false
});

Object.defineProperty(person, 'age', {
  writable: false
});

try {
  Object.defineProperty(person, 'age', {
    enumerable: false
  });
} catch (e) {
  console.log('Error: ' + e.message);
}

You can't modify a property more than twice
The age property is already non-writable
You can't change enumerable after initialization
When configurable is false, you can't change other attributes except writable true→false
The error occurs because of configurable: false restrictions: 1) When a property is non-configurable, most changes to its descriptor are prohibited, 2) You can only change writable from true to false as a one-way operation, 3) Attempting to change enumerable or any other attribute will fail, 4) The second defineProperty call works because it's only changing writable from true to false, 5) The third call fails because it's trying to change enumerable on a non-configurable property, 6) This restriction ensures that once a property is locked down as non-configurable, its behavior remains predictable, 7) This is part of how JavaScript implements true constants.

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