9.8 What property attributes do built-in constructs use?

The general rule (with few exceptions) for property attributes is:

• Properties of objects at the beginning of a prototype chain are usually writable, enumerable, and configurable.

• As described in the chapter on enumerability, most inherited properties are non-enumerable, to hide them from legacy constructs such as for-in loops. Inherited properties are usually writable and configurable.

9.8.1 Own properties created via assignment

const obj = {};
obj.prop = 3;
assert.deepEqual(
  Object.getOwnPropertyDescriptors(obj),
  {
    prop: {
      value: 3,
      writable: true,
      enumerable: true,
      configurable: true,
    }
  });

9.8.2 Own properties created via object literals

const obj = { prop: 'yes' };
assert.deepEqual(
  Object.getOwnPropertyDescriptors(obj),
  {
    prop: {
      value: 'yes',
      writable: true,
      enumerable: true,
      configurable: true
    }
  });

9.8.3 The own property .length of Arrays

The own property .length of Arrays is non-enumerable, so that it isn’t copied by Object.assign(), spreading, and similar operations. It is also non-configurable:

> Object.getOwnPropertyDescriptor([], 'length')
{ value: 0, writable: true, enumerable: false, configurable: false }
> Object.getOwnPropertyDescriptor('abc', 'length')
{ value: 3, writable: false, enumerable: false, configurable: false }

.length is a special data property, in that it is influenced by (and influences) other own properties (specifically, index properties).

9.8.4 Prototype properties of built-in classes

assert.deepEqual(
  Object.getOwnPropertyDescriptor(Array.prototype, 'map'),
  {
    value: Array.prototype.map,
    writable: true,
    enumerable: false,
    configurable: true
  });

9.8.5 Prototype properties and instance properties of user-defined classes

class DataContainer {
  accessCount = 0;
  constructor(data) {
    this.data = data;
  }
  getData() {
    this.accessCount++;
    return this.data;
  }
}
assert.deepEqual(
  Object.getOwnPropertyDescriptors(DataContainer.prototype),
  {
    constructor: {
      value: DataContainer,
      writable: true,
      enumerable: false,
      configurable: true,
    },
    getData: {
      value: DataContainer.prototype.getData,
      writable: true,
      enumerable: false,
      configurable: true,
    }
  });

Note that all own properties of instances of DataContainer are writable, enumerable, and configurable:

const dc = new DataContainer('abc')
assert.deepEqual(
  Object.getOwnPropertyDescriptors(dc),
  {
    accessCount: {
      value: 0,
      writable: true,
      enumerable: true,
      configurable: true,
    },
    data: {
      value: 'abc',
      writable: true,
      enumerable: true,
      configurable: true,
    }
  });