Interfaces

Interfaces

Interfaces define a contract — the shape an object must follow.

Defining an Interface

interface IsPerson {
    name: string;
    age: number;
    speak(text: string): void;
    spend(amount: number): number;
}

Using Interfaces

const me: IsPerson = {
    name: "Mario",
    age: 30,
    speak(text: string): void {
        console.log(text);
    },
    spend(amount: number): number {
        console.log("I spent", amount);
        return amount;
    },
};

const greetPerson = (person: IsPerson) => {
    console.log("Hello", person.name);
};

Interface vs Type Alias

Feature	Interface	Type Alias
Object shapes	✅	✅
Extends/implements	✅	Via intersection
Declaration merging	✅	❌
Union types	❌	✅

Key Takeaways

• Interfaces define contracts for object shapes
• Objects must have all properties defined in the interface
• Use interfaces for objects, type aliases for unions/primitives