TypeScript Static Methods and Properties

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++;
    }
}

In this example, the headcount is a static property that 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

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;
    }
}

In this example:

1. First, change the access modifier of the headcount static property from public to private so that its value cannot be changed outside of the class without creating a new Employee object.
2. Second, add the getHeadcount() static method that returns the value of the headcount 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

In practice, you will find the library that contains many static properties and methods like the Math object. It has PI, E, … static properties and abs(), round(), etc., static methods.

Conclusion

1. Static properties and methods are shared by all instances of a class.
2. Use the static keyword before a property or a method to make it static.