JavaScript ES5 Object Methods

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JavaScript ES5 Object Methods


ECMAScript 5 added a lot of new Object Methods to JavaScript.

ES5 New Object Methods

// Adding or changing an object property
Object.defineProperty(object, property, descriptor)
// Adding or changing many object properties
Object.defineProperties(object, descriptors)
// Accessing Properties
Object.getOwnPropertyDescriptor(object, property)
// Returns all properties as an array
Object.getOwnPropertyNames(object)
// Returns enumerable properties as an array
Object.keys(object)
// Accessing the prototype
Object.getPrototypeOf(object)
// Prevents adding properties to an object
Object.preventExtensions(object)
// Returns true if properties can be added to an object
Object.isExtensible(object)
// Prevents changes of object properties (not values)
Object.seal(object)
// Returns true if object is sealed
Object.isSealed(object)
// Prevents any changes to an object
Object.freeze(object)
// Returns true if object is frozen
Object.isFrozen(object)


Changing a Property Value

Syntax

Object.defineProperty(object, property, {value :

value

})

This example changes a property value:

Example

var person = {
firstName: “John”,
lastName : “Doe”,
language : “EN”
};
// Change a property
Object.defineProperty(person, “language”,
{value : “NO”});




Changing Meta Data

ES5 allows the following property meta data to 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

ES5 allows getters and setters to 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});


Listing All Properties

This example list all properties of an object:

Example

var person = {
firstName: “John”,
lastName : “Doe”,
language : “EN”
};
Object.defineProperty(person, “language”, {enumerable:false});
Object.getOwnPropertyNames(person);  // Returns an array of
properties


Listing Enumerable Properties

This example list only the enumerable properties of an object:

Example

var person = {
firstName: “John”,
lastName : “Doe”,
language : “EN”
};
Object.defineProperty(person, “language”, {enumerable:false});
Object.keys(person);  // Returns an array
of enumerable
properties


Adding a Property

This example adds a new property to an object:

Example

// Create an object:
var person = {
firstName: “John”,
lastName :
“Doe”,
language : “EN”
};
// Add a property
Object.defineProperty(person, “year”,
{value:”2008″});


Adding Getters and Setters

The

Object.defineProperty()

method can also be used to add Getters and
Setters:

Example

//Create an object
var person = {firstName:”John”, lastName:”Doe”};
// Define a getter
Object.defineProperty(person, “fullName”, {
get :
function () {return this.firstName + ” ” + this.lastName;}
});


A Counter Example

Example

// Define object
var 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;