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Classes in TypeScript

Classes model state and behavior with rich typing: fields, modifiers, inheritance, and interfaces.

Fields and constructors

class Point {
  x: number;
  y: number;
  constructor(x: number, y: number) { this.x = x; this.y = y; }
  move(dx: number, dy: number) { this.x += dx; this.y += dy; }
}

Parameter properties and readonly

class User {
  constructor(public readonly id: number, public name: string) {}
}

Shorthand for declaring and initializing fields from constructor params.

Access modifiers

  • public (default)
  • private (class‑private)
  • protected (visible to subclasses)
  • #private (runtime private fields, JS syntax)
class C { #secret = 1; private legacy = 2; }

Implements and extends

interface Drivable { drive(km: number): void }
class Car implements Drivable { drive(km: number) { /* ... */ } }

abstract class Shape { abstract area(): number }
class Circle extends Shape {
  constructor(public r: number) { super(); }
  area() { return Math.PI * this.r ** 2 }
}

this and polymorphic this

class Builder {
  setName(name: string): this { return this; }
  setAge(age: number): this { return this; }
}
class PersonBuilder extends Builder {}
const pb = new PersonBuilder().setName("Ada").setAge(30); // fluent API

Static members

class Id {
  static next = 1;
  static make() { return this.next++; }
}

Summary

  • Prefer readonly and parameter properties for concise, safe classes
  • Use interfaces for contracts; abstract classes for shared behavior and state
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