Classes in TypeScript | TypeScript

Classes in TypeScript are like JavaScript classes with added type annotations, access modifiers, and interface implementation.

Basic class syntax

TypeScript classes look like JavaScript classes with type annotations on properties and methods:

class User {
  id: number;
  name: string;
  email: string;

  constructor(id: number, name: string, email: string) {
    this.id = id;
    this.name = name;
    this.email = email;
  }

  displayName(): string {
    return this.name;
  }
}

Property initialization

Under strictPropertyInitialization, all class properties must be initialized in the constructor or have a default value:

class Counter {
  count: number = 0;        // default value
  label: string;            // must be initialized in constructor

  constructor(label: string) {
    this.label = label;
  }
}

Properties that are intentionally left uninitialized use the definite assignment assertion or undefined:

class Database {
  connection!: Connection;  // "I will set this before using it"

  async connect() {
    this.connection = await createConnection();
  }
}

The ! assertion tells TypeScript to trust you. Use it sparingly — it is an escape hatch.

Parameter properties

TypeScript provides shorthand syntax for declaring and initializing properties in the constructor:

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

// Equivalent to the longer form above
const user = new User(1, "Colin", "c@example.com");

The public (or private/readonly) keyword in the constructor parameter both declares the property and assigns it.

What to carry forward

class properties must be initialized under strict mode
parameter properties are shorthand for declaring and assigning in the constructor
the definite assignment assertion (!) is an escape hatch — use sparingly
class methods are typed like regular functions

The next lesson covers access modifiers and readonly properties.