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 metadata 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});
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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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