THE WORLD'S LARGEST WEB DEVELOPER SITE

JS Tutorial

JS HOME JS Introduction JS Where To JS Output JS Statements JS Syntax JS Comments JS Variables JS Operators JS Arithmetic JS Assignment JS Data Types JS Functions JS Objects JS Events JS Strings JS String Methods JS Numbers JS Number Methods JS Arrays JS Array Methods JS Array Sort JS Array Iteration JS Dates JS Date Formats JS Date Get Methods JS Date Set Methods JS Math JS Random JS Booleans JS Comparisons JS Conditions JS Switch JS Loop For JS Loop While JS Break JS Type Conversion JS Bitwise JS RegExp JS Errors JS Scope JS Hoisting JS Strict Mode JS this Keyword JS Let JS Const JS Arrow Function JS Classes JS Debugging JS Style Guide JS Best Practices JS Mistakes JS Performance JS Reserved Words JS Versions JS Version ES5 JS Version ES6 JS JSON

JS Forms

JS Forms Forms API

JS Objects

Object Definitions Object Properties Object Methods Object Accessors Object Constructors Object Prototypes Object ECMAScript 5

JS Functions

Function Definitions Function Parameters Function Invocation Function Call Function Apply Function Closures

JS HTML DOM

DOM Intro DOM Methods DOM Document DOM Elements DOM HTML DOM CSS DOM Animations DOM Events DOM Event Listener DOM Navigation DOM Nodes DOM Collections DOM Node Lists

JS Browser BOM

JS Window JS Screen JS Location JS History JS Navigator JS Popup Alert JS Timing JS Cookies

JS AJAX

AJAX Intro AJAX XMLHttp AJAX Request AJAX Response AJAX XML File AJAX PHP AJAX ASP AJAX Database AJAX Applications AJAX Examples

JS JSON

JSON Intro JSON Syntax JSON vs XML JSON Data Types JSON Parse JSON Stringify JSON Objects JSON Arrays JSON PHP JSON HTML JSON JSONP

JS vs jQuery

jQuery Selectors jQuery HTML jQuery CSS jQuery DOM

JS Examples

JS Examples JS HTML DOM JS HTML Input JS HTML Objects JS HTML Events JS Browser JS Exercises JS Quiz JS Certificate

JS References

JavaScript Objects HTML DOM Objects


JavaScript Object Properties


Properties are the most important part of any JavaScript object.


JavaScript Properties

Properties are the values associated with a JavaScript object.

A JavaScript object is a collection of unordered properties.

Properties can usually be changed, added, and deleted, but some are read only.


Accessing JavaScript Properties

The syntax for accessing the property of an object is:

objectName.property         // person.age

or

objectName["property"]      // person["age"]

or

objectName[expression]      // x = "age"; person[x]

The expression must evaluate to a property name.

Example 1

person.firstname + " is " + person.age + " years old.";
Try it Yourself »

Example 2

person["firstname"] + " is " + person["age"] + " years old.";
Try it Yourself »


JavaScript for...in Loop

The JavaScript for...in statement loops through the properties of an object.

Syntax

for (variable in object) {
  // code to be executed
}

The block of code inside of the for...in loop will be executed once for each property.

Looping through the properties of an object:

Example

var person = {fname:"John", lname:"Doe", age:25};

for (x in person) {
  txt += person[x];
}
Try it Yourself »

Adding New Properties

You can add new properties to an existing object by simply giving it a value.

Assume that the person object already exists - you can then give it new properties:

Example

person.nationality = "English";
Try it Yourself »

Deleting Properties

The delete keyword deletes a property from an object:

Example

var person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"};
delete person.age;   // or delete person["age"];
Try it Yourself »

The delete keyword deletes both the value of the property and the property itself.

After deletion, the property cannot be used before it is added back again.

The delete operator is designed to be used on object properties. It has no effect on variables or functions.

The delete operator should not be used on predefined JavaScript object properties. It can crash your application.


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)


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.