JavaScript objects


Example - Real Life Objects, Properties, and Methods

In real life, a car is an object.

A car has properties like weight and color, and methods like start and stop:

Object Properties Methods

car.name = Fiat

car.model = 500

car.weight = 850kg

car.color = white

car.start()

car.drive()

car.brake()

car.stop()

All cars have the same properties, but the property values differ from car to car.

All cars have the same methods, but the methods are performed at different times.


JavaScript Objects

You have already learned that JavaScript variables are containers for data values.

This code assigns a simple value (Fiat) to a variable named car:


                      var car = "Fiat";
                  

Objects are variables too. But objects can contain many values.

This code assigns many values (Fiat, 500, white) to a variable named car:


                      var car = {
                        type:"Fiat", 
                        model:"500", 
                        color:"white"
                      };
                  

The values are written as name:value pairs (name and value separated by a colon).

Note: JavaScript objects are containers for named values called properties or methods.


Object Definition

You define (and create) a JavaScript object with an object literal:


                      var person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"};
                

Spaces and line breaks are not important. An object definition can span multiple lines:


                      var person = {
                        firstName: "John",
                        lastName: "Doe",
                        age: 50,
                        eyeColor: "blue"
                      };
                  

Object Properties

The name:values pairs in JavaScript objects are called properties:

Property Property Value
firstName John
lastName Doe
age 50
eyeColor blue

Accessing Object Properties

You can access object properties in two ways:


                    objectName.propertyName
                

Example 1


                      // Create an object:
                      var person = {
                        firstName: "John",
                        lastName: "Doe",
                        id: 5566
                      };
                      // Display some data from the object:
                      document.getElementById("example-1").innerHTML =
                        person.firstName + " " + person.lastName;
                  

or:


                      objectName["propertyName"]
                  

Example 2


                      // Create an object:
                      var person = {
                        firstName: "John",
                        lastName : "Doe",
                        id     :  5566
                      };
                      // Display some data from the object:
                      document.getElementById("demo").innerHTML =
                      person["firstName"] + " " + person["lastName"];
                  

Object Methods

Objects can also have methods.

Methods are actions that can be performed on objects.

Methods are stored in properties as function definitions.

Property Property Value
firstName John
lastName Doe
age 50
eyeColor blue
fullName function() {return this.firstName + " " + this.lastName;}

Note: A method is a function stored as a property.


                      var person = {
                        firstName: "John",
                        lastName : "Doe",
                        id       : 5566,
                        fullName : function() {
                          return this.firstName + " " + this.lastName;
                        }
                      };
                  

The this Keyword

In a function definition, this refers to the "owner" of the function.

In the example above, this is the person object that "owns" the fullName function.

In other words, this.firstName means the firstName property of this object.

Note: You will learn more about this keyword in the next tutorials.


Accessing Object Methods

You access an object method with the following syntax:


                    objectName.methodName()
                

Example


                      name = person.fullName();
                

If you access a method without the () parentheses, it will return the function definition:


                      var person = {
                        firstName: "John",
                        lastName : "Doe",
                        id     : 5566,
                        fullName : function() {
                          return this.firstName + " " + this.lastName;
                        }
                      };
      
                      document.getElementById("demo").innerHTML = person.fullName;
                      // Result will be function definition : function() { return this.firstName + " " + this.lastName; }
                  

Exercises - test your knowledge

Exercise 1

Alert "John" by extracting information from the person object.


                      var person = {
                        firstName: "John",
                        lastName: "Doe"
                      };
                  

                var person = {
                    firstName: "John",
                    lastName: "Doe"
                };
                alert(person.firstName);
        

Exercise 2

Add the following property and value to the person object: country: Norway.



                      var person = {
                        firstName: "John",
                        lastName: "Doe",
                             
                      };
      
                  

                        var person = {
                          firstName: "John",
                          lastName: "Doe",
                          country: "Norway"     
                        };
                    

Exercise 3

Create an object called person with name = John, age = 50.
Then, access the object to alert("John is 50").


                      var person = {
                        name: "John",
                        age: 50
                        };
                      alert(person.name + " is " + person.age);
                  

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