Summary: in this tutorial, you will learn about the TypeScript static properties and methods.
Static properties
Unlike an instance property, a static property is shared among all instances of a class.
To declare a static property, you use the static
keyword. To access a static property, you use the className.propertyName
syntax. For example:
class Employee {
static headcount: number = 0;
constructor(
private firstName: string,
private lastName: string,
private jobTitle: string) {
Employee.headcount++;
}
}
Code language: TypeScript (typescript)
In this example, the headcount
is a static property that is initialized to zero. Its value is increased by 1 whenever a new object is created.
The following creates two Employee
objects and shows the value of the headcount
property. It returns two as expected.
let john = new Employee('John', 'Doe', 'Front-end Developer');
let jane = new Employee('Jane', 'Doe', 'Back-end Developer');
console.log(Employee.headcount); // 2
Code language: TypeScript (typescript)
Static methods
Similar to the static property, a static method is also shared across instances of the class. To declare a static method, you use the static
keyword before the method name. For example:
class Employee {
private static headcount: number = 0;
constructor(
private firstName: string,
private lastName: string,
private jobTitle: string) {
Employee.headcount++;
}
public static getHeadcount() {
return Employee.headcount;
}
}
Code language: TypeScript (typescript)
In this example:
- First, change the access modifier of the
headcount
static property frompublic
toprivate
so that its value cannot be changed outside of the class without creating a newEmployee
object. - Second, add the
getHeadcount()
static method that returns the value of theheadcount
static property.
To call a static method, you use the className.staticMethod()
syntax. For example:
let john = new Employee('John', 'Doe', 'Front-end Developer');
let jane = new Employee('Jane', 'Doe', 'Back-end Developer');
console.log(Employee.getHeadcount); // 2
Code language: TypeScript (typescript)
In practice, you will find a library that contains many static properties and methods like the Math
object. It has PI
, E
, … static properties and abs()
, round()
, etc., static methods.
Summary
- Static properties and methods are shared by all instances of a class.
- Use the
static
keyword before a property or a method to make it static.