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JS ObjectsLesson #231

JavaScript Object Management

// Adding or changing an object property

Property Management Methods

Example

Example Code

javascript

// 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:

Object.defineProperty(object, property, descriptor)

Adding a new Property

This example adds a new property to an object:

Example

Example Code

javascript

Try it Yourself

// 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:

Example

Example Code

javascript

Try it Yourself

// 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:

Example

Example Code

javascript

Try it Yourself

writable : true      // Property value can be changed

enumerable : true    // Property can be enumerated

configurable : true  // Property can be reconfigured

Example

Example Code

javascript

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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:

Example

Example Code

javascript

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// Defining a getter

get: function() { return language }

// Defining a setter

set: function(value) { language = value }

This example makes language read-only:

Example

Example Code

javascript

Try it Yourself

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

This example makes language not enumerable:

Example

Example Code

javascript

Try it Yourself

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:

Example

Example Code

javascript

Try it Yourself

// Create an Object

const person = {

  firstName: "John",

  lastName : "Doe",

  language : "EN"

};

// Get all Properties

Object.getOwnPropertyNames(person);

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

Example

Example Code

javascript

Try it Yourself

// 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

Object.keys(object)

List Enumerable Object Properties

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

Example

Example Code

javascript

Try it Yourself

// Create an Object

const person = {

  firstName: "John",

  lastName : "Doe",

  language : "EN"

};

// Change the "language" Property

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

// Get all Enumerable Properties

Object.keys(person);

The getOwnPropertyNames() method returns all properties. > The Object.keys() method returns all enumerable properties. > If you define object properties without enumerable:false, > the two methods will return the same.

Adding Getters and Setters

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

Example

Example Code

javascript

Try it Yourself

//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

Example

Example Code

javascript

Try it Yourself

// 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;}
});

// Play with the counter:

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.

Advanced Chapters: > JavaScript Object Definitions > JavaScript Object Advanced this > JavaScript Object Iterations > JavaScript Object Getters & Setters > JavaScript Object Management > JavaScript Object Protection > JavaScript Object Prototypes > JavaScript Object Reference