ECMAScript 5 (2009) added these Object management methods to 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
// Create an Object:
const person = {
firstName: "John",
lastName :
"Doe",
language : "EN"
};
// Add a Property
Object.defineProperty(person, "year",
{value:"2008"});
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Changing a Property Value
This example changes a property value:
Example
// Create an Object:
const person = {
firstName: "John",
lastName : "Doe",
language : "EN"
};
// Change a Property
Object.defineProperty(person, "language",
{value : "NO"});
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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 getOwnProperyNames()
The Object.getOwnProperyNames() method can:
- List object properties
Syntax
Object.getOwnProperyNames(object)
List all Object Properties
This example gets all properties of an object:
Example
// Create an Object
const person = {
firstName: "John",
lastName : "Doe",
language : "EN"
};
// Get all Properties
Object.getOwnPropertyNames(person);
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Object.getOwnProperyNames() will also list properties that is not enumerable:
Example
// 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);
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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.getOwnProperyNames():
Example
// 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);
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Note
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
//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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