Creating subclasses with extends

In JavaScript you can create subclasses (as a child class of another class) with the extends keyword.
The subclass inherits and can extend the properties and methods defined in parent class.
If the subclass has no constructor(), it will be used the constructor() of the parent class.


- Example:
<div id='ex_res'>Shows response</div>

<script>
//Base class
class User {
 constructor(name){
 //set property
 this.name = name;
 }

 //method
 sayHi(){
 return 'Hello '+ this.name;
 }
}

//Subclass
class Gamer extends User {
 //method that uses the name property defined in parent class
 sayBye(){
 return 'Goodbye '+ this.name;
 }
}

//Usage

//set an object instance of subclass
//it needs an argument because it is used the parent constructor()
let ob_gbr = new Gamer('MarPlo');

//calls the sayHi() method defined in the parent class (User)
var hi = ob_gbr.sayHi();

//calls the method defined in subclass
var bye = ob_gbr.sayBye();

//shows the value of 'hi' and 'bye' in Div #ex_res
document.getElementById('ex_res').innerHTML ='<h3>'+hi+'<br>'+bye+'</h3>';
</script>

Class Inheritance, super

The subclass inherits the properties and methods defined in parent class (including static methods).
If there is a constructor present in subclass, it needs to first call super() before using "this".
To rewrite in subclass some properties and methods from parent class, just define them again in subclass.


- Example:
<div id='ex_res'>Shows response</div>

<script>
//Base class
class Parent {
 constructor(name){
 //set property
 this.name = name;
 }

 //method
 sayHi(){
 return 'Hello '+ this.name;
 }
}

//Subclass
class Child extends Parent {
 constructor(name){
 //apply super to include parent constructor and pass in the name parameter
 super(name);

 //here you can rewrite or define properties

 this.site ='CoursesWeb.net';
 }

 //rewrite method from parent
 sayHi(){
 return 'From Subclass, site: '+ this.site +'<br>Hello '+ this.name;
 }
}

//Usage

//set an object instance of subclass Child
let obj = new Child('MarPlo');

//calls the sayHi() method, rewrited in child class
var hi = obj.sayHi();

//shows the value of 'hi' in Div #ex_res
document.getElementById('ex_res').innerHTML ='<h2>'+hi+'</h2>';
</script>

Class Methods with super

The super keyword is used to call corresponding methods of super class. It is used when you don't want to totally replace the parent method, but to add new instructions to it.
Classes provide "super" keyword for that.


- Example:
<div id='ex_res'>Shows response</div>

<script>
//Base class
class Parent {
 constructor(name){
 //set property
 this.name = name;
 }

 //method
 sayHi(){
 return 'Hello '+ this.name;
 }
}

//Subclass
class Child extends Parent {
 constructor(name, site){
 //apply super to include parent constructor and pass in the name parameter
 super(name);

 //here you can rewrite or define properties
 this.site = site;
 }

 //reuse method from parent
 sayHi(){
 var hi = super.sayHi(); //gets data returned by parent method

 //here you can add new instructions

 return hi +'<br>From: '+ this.site;
 }
}

//Usage

//set an object instance of subclass Child
let obj = new Child('MarPlo', 'CoursesWeb.net');

//calls the sayHi() method redefined in the Child class
var hi = obj.sayHi();

//shows the value of 'hi' in Div #ex_res
document.getElementById('ex_res').innerHTML ='<h2>'+hi+'</h2>';
</script>

super in static methods

The "super" keyword can be applied in static methods too.
- Example:
<script>
//Base class
class Parent {
 //static method
 static sayHi(name){
 alert('Hello '+ name +' from parent');
 }
}

//Subclass
class Child extends Parent {
 //redefine the static method
 static sayHi(name, name2){
 super.sayHi(name); //include the method from parent
 alert('Hello '+ name2 +' from subclass');
 }
}

//calls the static method from subclass
Child.sayHi('MarPlo', 'Gamv');
</script>

Daily Test with Code Example

HTML
CSS
JavaScript
PHP-MySQL
Which meta tag provides a short description of the page?
<meta content="..."> <meta description="..."> <meta http-equiv="...">
<meta name="description" content="70-160 characters that describes the content of the page" />
Which CSS property is used to stop the wrapping effect of the "float"?
clear text-align position
#some_id {
  clear: both;
}
Click on the method which gets an array with all the elements in the document that have a specified tag name.
getElementsByName() getElementById() getElementsByTagName()
var divs = document.getElementsByTagName("div");
var nr_divs = divs.length;
alert(nr_divs);
Indicate the PHP function which returns the number of elements in array.
is_[) count() strlen()
$arr =[7, 8, "abc", 10);
$nri = count($arr);
echo $nri;        // 4
Subclass with extends and Inheritance

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