object-descriptors.md

Object Descriptors

Common Keys	Data Descriptor	Accessor Descriptor
configurable (=false)	value (=undefined)	get (=undefined)
enumerable (=false)	writable (=false)	set (=undefined)

• if a descriptor has neither of value, writable, get and set keys, it is treated as a data descriptor
• if a descriptor has both [value or writable] and [get or set] keys, TypeError is thrown

Data Descriptor Example

const obj = {};

Object.defineProperties(obj, {
  a: {
    enumerable: true,
    value: 1
  },
  b: {
    enumerable: false,
    value: 2
  },
  c: {
    value: 3
  }
});

{ ...obj }
// {a: 1}

for (let k in obj) console.log(k);
// a

Object.keys(obj);
// ['a']

Accessor Descriptor Example

function LoggedValue(obj) {
  const logs = [];
  let _value;

  Object.defineProperty(this, 'value', {
    get() {
      return _value;
    },
    set(value) {
      _value = value;
      logs.push(_value);
    }
  });

  this.logs = () => logs;
}

const record = new LoggedValue();

record.value = 10;
record.value = 20;

record.logs();
// [10, 20]

Getting Descriptors

Object.getOwnPropertyDescriptor()

const obj = { a: 1 };
Object.seal(obj);

Object.getOwnPropertyDescriptor(obj, 'a');
// {value: 1, writable: true, enumerable: true, configurable: false}

Object.getOwnPropertyDescriptors()

const obj = { a: 1 };
Object.seal(obj);

Object.getOwnPropertyDescriptors(obj);
// {a: {
//   configurable: false,
//   enumerable: true,
//   value: 1,
//   writable: true
// }}

Getting Property Getter

const foo = {
  bar: 1,
  get baz() {
    return 2;
  }
};

const { get: getter } = Object.getOwnPropertyDescriptor(foo, 'baz');
getter
// function baz() { return 2; }

Deprecated and non-standard way of getting a getter:
foo.__lookupGetter__('baz')