9.6 Use cases for Object.getOwnPropertyDescriptors()

Object.getOwnPropertyDescriptors() helps us with two use cases, if we combine it with Object.defineProperties() or Object.create().

9.6.1 Use case: copying properties into an object

Since ES6, JavaScript already has had a tool method for copying properties: Object.assign(). However, this method uses simple get and set operations to copy a property whose key is key:

target[key] = source[key];

That means that it only creates a faithful copy of a property if:

• Its attribute writable is true and its attribute enumerable is true (because that’s how assignment creates properties).
• It is a data property.

The following example illustrates this limitation. Object source has a setter whose key is data.

const source = {
  set data(value) {
    this._data = value;
  }
};
// Because there is only a setter, property `data` exists,
// but has the value `undefined`.
assert.equal('data' in source, true);
assert.equal(source.data, undefined);

If we use Object.assign() to copy property data, then the accessor property data is converted to a data property:

const target1 = {};
Object.assign(target1, source);
assert.deepEqual(
  Object.getOwnPropertyDescriptor(target1, 'data'),
  {
    value: undefined,
    writable: true,
    enumerable: true,
    configurable: true,
  });
// For comparison, the original:
const desc = Object.getOwnPropertyDescriptor.bind(Object);
assert.deepEqual(
  Object.getOwnPropertyDescriptor(source, 'data'),
  {
    get: undefined,
    set: desc(source, 'data').set,
    enumerable: true,
    configurable: true,
  });

Fortunately, using Object.getOwnPropertyDescriptors() together with Object.defineProperties() does faithfully copy the property data:

const target2 = {};
Object.defineProperties(
  target2, Object.getOwnPropertyDescriptors(source));
assert.deepEqual(
  Object.getOwnPropertyDescriptor(target2, 'data'),
  {
    get: undefined,
    set: desc(source, 'data').set,
    enumerable: true,
    configurable: true,
  });

9.6.1.1 Pitfall: copying methods that use super

A method that uses super is firmly connected with its home object (the object it is stored in). There is currently no way to copy or move such a method to a different object.

9.6.2 Use case for Object.getOwnPropertyDescriptors(): cloning objects

Shallow cloning is similar to copying properties, which is why Object.getOwnPropertyDescriptors() is a good choice here, too.

To create the clone, we use Object.create():

const original = {
  set data(value) {
    this._data = value;
  }
};
const clone = Object.create(
  Object.getPrototypeOf(original),
  Object.getOwnPropertyDescriptors(original));
assert.deepEqual(original, clone);

For more information on this topic, see §6 “Copying objects and Arrays”.