TypeScript: Interfaces & Custom Types

What you will learn

How to define object shapes with interface, how optional properties work, and how extend lets you compose interfaces.

Defining an interface

Interfaces describe the shape -- the expected properties and their types -- of an object:

interface User {
    id: number;
    name: string;
    email?: string;   // optional property (can be absent)
    readonly createdAt: Date;  // read-only after creation
}

function registerUser(newUser: User): void {
    // ...
}

registerUser({ id: 1, name: 'Alice', createdAt: new Date() });

Extending interfaces

interface Animal {
    name: string;
    speak(): string;
}

interface Dog extends Animal {
    breed: string;
}

const myDog: Dog = {
    name: 'Rex',
    breed: 'German Shepherd',
    speak() { return 'Woof!'; }
};

Interface vs Type alias

You can also use type to define shapes:

type Point = {
    x: number;
    y: number;
};

The main difference: interface can be merged (declared multiple times, TS combines them) and extended. type cannot be reopened but can define primitives and unions.

Common mistakes

• Forgetting that interfaces describe minimal requirements -- extra properties cause errors unless you use index signatures
• Leaving required properties uninitialized when creating an object

Quick check below!