9.3 Retrieving descriptors for properties

9.3.1 Object.getOwnPropertyDescriptor(): retrieving a descriptor for a single property

Consider the following object:

  1. const legoBrick = {
  2.   kind: 'Plate 1x3',
  3.   color: 'yellow',
  4.   get description() {
  5.     return `${this.kind} (${this.color})`;
  6.   },
  7. };

Let’s first get a descriptor for the data property .color:

  1. assert.deepEqual(
  2.   Object.getOwnPropertyDescriptor(legoBrick, 'color'),
  3.   {
  4.     value: 'yellow',
  5.     writable: true,
  6.     enumerable: true,
  7.     configurable: true,
  8.   });

This is what the descriptor for the accessor property .description looks like:

  1. const desc = Object.getOwnPropertyDescriptor.bind(Object);
  2. assert.deepEqual(
  3.   Object.getOwnPropertyDescriptor(legoBrick, 'description'),
  4.   {
  5.     get: desc(legoBrick, 'description').get, // (A)
  6.     set: undefined,
  7.     enumerable: true,
  8.     configurable: true
  9.   });

Using the utility function desc() in line A ensures that .deepEqual() works.

9.3.2 Object.getOwnPropertyDescriptors(): retrieving descriptors for all properties of an object

  1. const legoBrick = {
  2.   kind: 'Plate 1x3',
  3.   color: 'yellow',
  4.   get description() {
  5.     return `${this.kind} (${this.color})`;
  6.   },
  7. };
  9. const desc = Object.getOwnPropertyDescriptor.bind(Object);
  10. assert.deepEqual(
  11.   Object.getOwnPropertyDescriptors(legoBrick),
  12.   {
  13.     kind: {
  14.       value: 'Plate 1x3',
  15.       writable: true,
  16.       enumerable: true,
  17.       configurable: true,
  18.     },
  19.     color: {
  20.       value: 'yellow',
  21.       writable: true,
  22.       enumerable: true,
  23.       configurable: true,
  24.     },
  25.     description: {
  26.       get: desc(legoBrick, 'description').get, // (A)
  27.       set: undefined,
  28.       enumerable: true,
  29.       configurable: true,
  30.     },
  31.   });

Using the helper function desc() in line A ensures that .deepEqual() works.