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JavaScript ES5 Object Methods

ECMAScript 5 (2009) added a lot of new Object Methods to JavaScript.

Managing Objects

// Create object with an existing object as prototype
Object.create()

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

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

// Accessing Properties
Object.getOwnPropertyDescriptor(object, property)

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

// Accessing the prototype
Object.getPrototypeOf(object)

// Returns enumerable properties as an array
Object.keys(object)

Protecting Objects

// 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

const person = {
  firstName: "John",
  lastName : "Doe",
  language : "EN"
};

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

const person = {
  firstName: "John",
  lastName : "Doe",
  language : "EN"
};

Object.defineProperty(person, "language", {enumerable:false});
Object.getOwnPropertyNames(person);  // Returns an array of properties
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Listing Enumerable Properties

This example list only the enumerable properties of an object:

Example

const person = {
  firstName: "John",
  lastName : "Doe",
  language : "EN"
};

Object.defineProperty(person, "language", {enumerable:false});
Object.keys(person);  // Returns an array of enumerable properties
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Adding a Property

This example adds a new property to an object:

Example

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

// Add a property
Object.defineProperty(person, "year", {value:"2008"});
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Adding Getters and Setters

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

Example

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

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

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;}
});

// Play with the counter:
obj.reset;
obj.add = 5;
obj.subtract = 1;
obj.increment;
obj.decrement;
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