The class declaration creates a
of a new class to a given name.
You can also define classes using the class expression.
Syntax
class name {
// class body
}
class name extends otherName {
// class body
}
Description
The class body of a class declaration is executed in strict mode. The class declaration is very similar to let:
classdeclarations are scoped to blocks as well as functions.classdeclarations can only be accessed after the place of declaration is reached (see temporal dead zone). For this reason,classdeclarations are commonly regarded as non-hoisted (unlike function declarations).classdeclarations do not create properties on globalThis when declared at the top level of a script (unlike function declarations).classdeclarations cannot be redeclared by any other declaration in the same scope.
Outside the class body, class declarations can be re-assigned like let, but you should avoid doing so. Within the class body, the binding is constant like const.
class Foo {
static {
Foo = 1; // TypeError: Assignment to constant variable.
}
}
class Foo2 {
bar = (Foo2 = 1); // TypeError: Assignment to constant variable.
}
class Foo3 {}
Foo3 = 1;
console.log(Foo3); // 1
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;
}
}