Abstract Classes

Syntax

abstract class Name {
  abstract method(): type;
}

Examples

abstract class Shape {
  abstract getArea(): number; // no implementation - subclasses MUST provide one

  describe(): string {
    // a concrete method, shared and inherited as-is by every subclass
    return `This shape has an area of ${this.getArea()}`;
  }
}

// const shape = new Shape(); // Error - cannot instantiate an abstract class

class Circle extends Shape {
  constructor(private radius: number) {
    super();
  }
  getArea(): number {
    return Math.PI * this.radius ** 2;
  }
}

const circle = new Circle(5);
console.log(circle.describe());

Best practices

• Use abstract classes when related classes should share both a common contract (abstract methods) and some actual shared implementation (concrete methods)
• Use a plain interface instead when you only need to enforce a shape/contract with no shared implementation at all
• Remember abstract classes can never be instantiated directly - attempting new AbstractClass() is always a compile error
• Keep abstract method signatures focused and minimal - the fewer methods subclasses are forced to implement, the easier the class family is to extend

At a glance

Purpose

Static types for JavaScript

File extension

.ts · .tsx

Runs in

Compiled to JavaScript

Usually used with

JavaScript and its ecosystem

Specifications & further reading

• TypeScript Handbook

Related TypeScript documentation