Interfaces & Classes

As your programs grow, you need ways to describe the shape of your data and organize code into reusable structures. Interfaces and classes are TypeScript's primary tools for this.

Interfaces

An interface defines the shape of an object — what properties it should have and what types those properties should be:

interface User {
  name: string;
  email: string;
  age: number;
}

const alice: User = {
  name: "Alice",
  email: "[email protected]",
  age: 30,
};

Optional & readonly properties

interface User {
  name: string;          // required
  email: string;         // required
  age?: number;          // optional — the ? makes it optional
  readonly id: number;   // readonly — cannot be changed after creation
}

const user: User = {
  name: "Alice",
  email: "[email protected]",
  id: 1,
};

user.age = 30;       // ✅ Optional — can be set
user.id = 2;         // ❌ Error: Cannot assign to 'id' because it is read-only

Interfaces vs type aliases

Both interface and type can define object shapes. The main difference:

• → Interfaces can be extended and merged — great for large, evolving systems.
• → Type aliases are more flexible — they can represent unions, primitives, and tuples.
• → In practice, use interfaces for object shapes and types for everything else.

Classes

Classes combine data (properties) and behavior (methods) into a single structure. TypeScript adds access modifiers and interface implementation:

class Student implements User {
  name: string;
  email: string;
  private gpa: number;

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

  // Method
  getSummary(): string {
    return `${this.name} (${this.email}) — GPA: ${this.gpa}`;
  }
}

const s = new Student("Alice", "[email protected]", 3.8);
console.log(s.getSummary());

Access modifiers

• → public — accessible from anywhere (default).
• → private — accessible only within the class.
• → protected — accessible within the class and its subclasses.

Quiz