class Keyword
| Site: | Saylor Academy |
| Course: | PRDV401: Introduction to JavaScript I |
| Book: |
class Keyword
|
| Printed by: | Guest user |
| Date: | Tuesday, 13 May 2025, 8:54 PM |
class Keyword
The class declaration creates a new class with a given name using prototype-based inheritance.
Try It
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Syntax
class name [extends otherName] {
// class body
}
Description
The class body of a class declaration is executed in strict mode. The
constructor method is optional.
Class declarations are not hoisted (unlike function declarations).
Examples
A simple class declaration
In the following example, we first define a class named Rectangle, then
extend it to create a class named FilledRectangle.
Note that super(), used in the constructor, can only be used
in constructors, and must be called before the this
keyword can be used.
class Rectangle {
constructor(height, width) {
this.name = 'Rectangle';
this.height = height;
this.width = width;
}
}
class FilledRectangle extends Rectangle {
constructor(height, width, color) {
super(height, width);
this.name = 'Filled rectangle';
this.color = color;
}
}
Attempting to declare a class twice
Re-declaring a class using the class declaration throws a SyntaxError.
class Foo {};
class Foo {}; // Uncaught SyntaxError: Identifier 'Foo' has already been declared
The same error is thrown when a class has been defined before using the class expression.
let Foo = class {};
class Foo {}; // Uncaught SyntaxError: Identifier 'Foo' has already been declared