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JavaScript Property Descriptors

Published Mar 27 2019

Any object in JavaScript has a set of properties, and each of these properties has a descriptor.

This is an object that defines a property behavior and own properties.

Many Object static methods interact with it. Those methods include:

• Object.create()
• Object.defineProperties()
• Object.defineProperty()
• Object.getOwnPropertyDescriptor()
• Object.getOwnPropertyDescriptors()

Here is an example of a property descriptor object:

{
  value: 'Something'
}

This is the simplest one. value is the property value, in a key-value definition. This key is defined as the object key, when you define this property in an object:

{
  breed: {
    value: 'Siberian Husky'
  }
}

Example:

const animal = {}
const dog = Object.create(animal, {
  breed: {
    value: 'Siberian Husky'
  }
});
console.log(dog.breed) //'Siberian Husky'

You can pass additional properties to define each different object property:

• value: the value of the property
• writable: true the property can be changed
• configurable: if false, the property cannot be removed nor any attribute can be changed, except its value
• enumerable: true if the property is enumerable
• get: a getter function for the property, called when the property is read
• set: a setter function for the property, called when the property is set to a value

writable, configurable and enumerable set the behavior of that property. They have a boolean value, and by default those are all false.

Example:

const animal = {}
const dog = Object.create(animal, {
  breed: {
    value: 'Siberian Husky',
    writable: false
  }
});
console.log(dog.breed) //'Siberian Husky'
dog.breed = 'Pug' //TypeError: Cannot assign to read only property 'breed' of object '#<Object>'

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