JavaScript Object Properties

Property Management Methods

// Adding or changing an object property
Object.defineProperty(object, property, descriptor)

// Adding or changing object properties
Object.defineProperties(object, descriptors)

// Accessing a Property
Object.getOwnPropertyDescriptor(object, property)

// Accessing Properties
Object.getOwnPropertyDescriptors(object)

// Returns all properties as an array
Object.getOwnPropertyNames(object)

// Accessing the prototype
Object.getPrototypeOf(object)

JavaScript Object.defineProperty()

The Object.defineProperty() method can be used to:

• Adding a new property to an object
• Changing property values
• Changing property metadata
• Changing object getters and setters

Syntax :

Adding a new Property

This example adds a new property to an object:

// Create an Object:
const person = {
  firstName: "John",
  lastName : "Doe",
  language : "EN"
};

// Add a Property
Object.defineProperty(person, "year", {value:"2008"});

Changing a Property Value

This example changes a property value:

// Create an Object:
const person = {
  firstName: "John",
  lastName : "Doe",
  language : "EN"
};

// Change a Property
Object.defineProperty(person, "language", {value : "NO"});

Property Attributes

All properties have a name. In addition they also have a value.

The value is one of the property’s attributes.

Other attributes are: enumerable, configurable, and writable.

These attributes define how the property can be accessed (is it readable?, is it writable?)

In JavaScript, all attributes can be read, but only the value attribute can be changed (and only if the property is writable).

( ECMAScript 5 has methods for both getting and setting all property attributes)

Changing Meta Data

The following property meta data can be changed :

writable : true      // Property value can be changed
enumerable : true    // Property can be enumerated
configurable : true  // Property can be reconfigured
writable : false     // Property value can not be changed
enumerable : false   // Property can be not enumerated
configurable : false // Property can be not reconfigured

Getters and setters can also be changed:

// Defining a getter
get: function() { return language }
// Defining a setter
set: function(value) { language = value }

This example makes language read-only:

Object.defineProperty(person, "language", {writable:false});

This example makes language not enumerable:

Object.defineProperty(person, "language", {enumerable:false});

JavaScript getOwnPropertyNames()

The Object.getOwnPropertyNames() method can:

• List object properties

Syntax

Object.getOwnPropertyNames(object)

List all Object Properties

This example gets all properties of an object:

// Create an Object
const person = {
  firstName: "John",
  lastName : "Doe",
  language : "EN"
};

// Get all Properties
Object.getOwnPropertyNames(person);

Object.getOwnPropertyNames() will also list properties that is not enumerable:

// Create an Object
const person = {
  firstName: "John",
  lastName : "Doe",
  language : "EN"
};

// Set the language Property not enumerable
Object.defineProperty(person, "language", {enumerable:false});

// Get all Properties
Object.getOwnPropertyNames(person);

JavaScript Object.keys()

The Object.keys() method can:

• List enumerable object properties

Syntax

List Enumerable Object Properties

This example uses Object.keys() insted of Object.getOwnPropertyNames():

// Create an Object
const person = {
  firstName: "John",
  lastName : "Doe",
  language : "EN"
};

Adding Getters and Setters

The Object.defineProperty() method can also be used to add Getters and Setters:

//Create an object
const person = {firstName:"John", lastName:"Doe"};

// Define a getter
Object.defineProperty(person, "fullName", {
  get: function () {return this.firstName + " " + this.lastName;}
});

A Counter Example

// Define object
const obj = {counter:0};

// Define setters
Object.defineProperty(obj, "reset", {
  get : function () {this.counter = 0;}
});
Object.defineProperty(obj, "increment", {
  get : function () {this.counter++;}
});
Object.defineProperty(obj, "decrement", {
  get : function () {this.counter--;}
});
Object.defineProperty(obj, "add", {
  set : function (value) {this.counter += value;}
});
Object.defineProperty(obj, "subtract", {
  set : function (i) {this.counter -= i;}
});

obj.reset;
obj.add = 5;
obj.subtract = 1;
obj.increment;
obj.decrement;

Prototype Properties

JavaScript objects inherit the properties of their prototype.

The delete keyword does not delete inherited properties, but if you delete a prototype property, it will affect all objects inherited from the prototype.