Extending Interfaces

Interfaces can extend other interfaces to build complex types through inheritance, promoting code reuse and maintainability.

Extending Interfaces

Use the extends keyword to inherit properties from parent interfaces:

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Multiple Inheritance

TypeScript interfaces can extend multiple interfaces at once:

interface Timestamped {
    createdAt: Date;
    updatedAt: Date;
}

interface Named {
    firstName: string;
    lastName: string;
}

// Extends both interfaces
interface User extends Named, Timestamped {
    email: string;
}

let user: User = {
    firstName: "Alice",
    lastName: "Johnson",
    email: "[email protected]",
    createdAt: new Date(),
    updatedAt: new Date()
};

When to Extend Interfaces

• Shared properties: Multiple types need common fields
• Hierarchies: Base type with specialized variants
• Composition: Combine multiple concerns
• API responses: Base response + specific data

Interface Extension Patterns

Common Mistakes

interface A {
    value: string;
}

interface B {
    value: number;
}

// ✗ Wrong - conflicting types for 'value'
interface C extends A, B { }  // Error!

// ✓ Correct - compatible types
interface A {
    value: string | number;
}

interface B {
    value: number;
}

interface C extends A, B { }  // OK

interface Animal {
    nameAnimal: string;
}

interface Dog extends Animal {
    breed: string;
}

// ✗ Wrong - missing 'nameAnimal'
let dog: Dog = {
    breed: "Labrador"
};

// ✓ Correct
let dog: Dog = {
    nameAnimal: "Buddy",
    breed: "Labrador"
};

Summary

What's Next?

Congratulations! You've completed Module 2. Next: Classes & OOP!