9.4 Defining properties via descriptors

If we define a property with the key k via a property descriptor propDesc, then what happens depends:

  • If there is no property with key k, a new own property is created that has the attributes specified by propDesc.
  • If there is a property with key k, defining changes the property’s attributes so that they match propDesc.

9.4.1 Object.defineProperty(): defining single properties via descriptors

First, let us create a new property via a descriptor:

  1. const car = {};
  3. Object.defineProperty(car, 'color', {
  4.   value: 'blue',
  5.   writable: true,
  6.   enumerable: true,
  7.   configurable: true,
  8. });
  10. assert.deepEqual(
  11.   car,
  12.   {
  13.     color: 'blue',
  14.   });

Next, we change the kind of a property via a descriptor; we turn a data property into a getter:

  1. const car = {
  2.   color: 'blue',
  3. };
  5. let readCount = 0;
  6. Object.defineProperty(car, 'color', {
  7.   get() {
  8.     readCount++;
  9.     return 'red';
  10.   },
  11. });
  13. assert.equal(car.color, 'red');
  14. assert.equal(readCount, 1);

Lastly, we change the value of a data property via a descriptor:

  1. const car = {
  2.   color: 'blue',
  3. };
  5. // Use the same attributes as assignment:
  6. Object.defineProperty(
  7.   car, 'color', {
  8.     value: 'green',
  9.     writable: true,
  10.     enumerable: true,
  11.     configurable: true,
  12.   });
  14. assert.deepEqual(
  15.   car,
  16.   {
  17.     color: 'green',
  18.   });

We have used the same property attributes as assignment.

9.4.2 Object.defineProperties(): defining multiple properties via descriptors

Object.defineProperties() is the multi-property version of `Object.defineProperty():

  1. const legoBrick1 = {};
  2. Object.defineProperties(
  3.   legoBrick1,
  4.   {
  5.     kind: {
  6.       value: 'Plate 1x3',
  7.       writable: true,
  8.       enumerable: true,
  9.       configurable: true,
  10.     },
  11.     color: {
  12.       value: 'yellow',
  13.       writable: true,
  14.       enumerable: true,
  15.       configurable: true,
  16.     },
  17.     description: {
  18.       get: function () {
  19.         return `${this.kind} (${this.color})`;
  20.       },
  21.       enumerable: true,
  22.       configurable: true,
  23.     },
  24.   });
  26. assert.deepEqual(
  27.   legoBrick1,
  28.   {
  29.     kind: 'Plate 1x3',
  30.     color: 'yellow',
  31.     get description() {
  32.       return `${this.kind} (${this.color})`;
  33.     },
  34.   });