TypeScript Academy · Lesson

Interface Extension and Merging

Extend interfaces and understand declaration merging.

Interface Extension and Merging is a free TypeScript Academy lesson on CoddyKit — lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the TypeScript Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Welcome

Interfaces can extend other interfaces to build on existing shapes, and TypeScript merges multiple declarations of the same interface automatically.

Extending an Interface

Use the `extends` keyword to create a new interface that includes all properties from the parent.

interface Animal { name: string; }
interface Dog extends Animal {
  breed: string;
}
const rex: Dog = { name: 'Rex', breed: 'Labrador' };

Extending Multiple Interfaces

An interface can extend multiple interfaces at once, combining all their properties.

interface Flyable { fly(): void; }
interface Swimmable { swim(): void; }
interface Duck extends Flyable, Swimmable {
  quack(): void;
}

Override Properties When Extending

An extended interface can override a property from the parent, but the new type must be assignable to the parent type.

interface Base { value: string | number; }
interface Derived extends Base { value: string; } // OK — string is narrower
// interface Bad extends Base { value: boolean; } // Error

Declaration Merging

When you declare two interfaces with the same name, TypeScript merges them into a single interface with all properties.

interface Window { title: string; }
interface Window { location: Location; }
// Merged: Window has both title and location

Augmenting Library Types

Declaration merging is used to extend third-party types. Add properties to Express's Request interface, for example.

// express.d.ts
declare namespace Express {
  interface Request {
    user?: User;
  }
}

Interface Extending type Alias

An interface can extend a type alias as long as the alias is an object type.

type Named = { name: string };
interface User extends Named { email: string; }

Nested Extension Chains

Extension chains can be multiple levels deep.

interface A { a: number; }
interface B extends A { b: number; }
interface C extends B { c: number; }
const obj: C = { a: 1, b: 2, c: 3 };

extends vs implements

`extends` is for interface-to-interface or class-to-class inheritance. `implements` is for a class to fulfill an interface contract.

interface Serializable { serialize(): string; }
class Model implements Serializable {
  serialize() { return JSON.stringify(this); }
}

Merging Adds to All Usages

When interfaces merge, all existing code that uses that interface automatically gets the new properties, which is why library augmentation works.

Merging vs Extending: When to Use Each

Use `extends` to create a new interface based on an existing one. Use merging (same name) to add to an interface from a separate file (like augmentation).

Quick Check

Which TypeScript feature allows you to add properties to an interface from a third-party library?

Recap

Interfaces extend other interfaces with `extends`, combining their shapes. Declaration merging automatically combines same-name interfaces. This enables library augmentation without modifying third-party code.

Frequently asked questions

Is the “Interface Extension and Merging” lesson free?

Yes — the full text of “Interface Extension and Merging” is free to read here on the web, and the TypeScript Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the TypeScript Academy course, upgrade to CoddyKit PRO.

What will I learn in “Interface Extension and Merging”?

Extend interfaces and understand declaration merging. You practise TypeScript Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start TypeScript Academy?

No prior experience is required. TypeScript Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Interface Extension and Merging” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this TypeScript Academy lesson?

Yes. Every TypeScript Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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