javascript

Chapter 22: The Evolution of JavaScript
extends keyword to indicate that they inherit from another class. As with other OO languages, they
can only inherit from one class. Here is an example:
class Person {
var name: string;
var age: int;
}
function sayName(){
alert(this.name);
}
class Employee extends Person {
var job: string;
}
var me = new Employee();
alert(me is Person);
alert(me is Employee);
//true
//true
This code defines two classes: Person and Employee , with the latter inheriting from the former. Unlike
ECMAScript 3, no additional work is necessary to ensure that the subclass inherits all properties and
methods from the superclass. Additionally, extending a class ensures that instance of a subclass also
has a type of the superclass, meaning that the statements me is Person and me is Employee both
are true.
Overriding superclass Methods
To override a superclass method, you must explicitly do so using the override tag as shown in the
following example. Attempting to define a method of the same name that exists in the superclass will
cause an error.
class Person {
var name: string;
var age: int;
}
function sayName(){
alert(this.name);
}
class Employee extends Person {
var job: string;
}
override function sayName(){
alert(this.name + “:” + this.job);
}
This example overrides the sayName() method in the Employee class to output the name and job
properties.
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