Introduction to Interfaces

An Interface in TypeScript is a powerful structure that exclusively defines the conceptual shape of an object or a class map. They vanish completely when the code is transpiled to .js.

Defining an Interface

interface User {
    id: number;
    name: string;
    email: string;
    age?: number; // Optional property
}

// Creating an object bound strictly to the User interface
const newSubmit: User = {
    id: 1,
    name: "Alice",
    email: "alice@test.com"
    // 'age' omitted because it's optional
};

readonly Properties

If an object property should never be altered after the object initializes (like database IDs), prefix it with readonly.

interface Product {
    readonly _id: string; // Internal Database Mongo ID
    title: string;
    price: number;
}

const item: Product = {
    _id: "6a89bxo9p",
    title: "Keyboard",
    price: 45
};

item.price = 50;  // Works fine
// item._id = "new_id"; // TS ERROR: Cannot assign to '_id' because it is a read-only property.

When to use Interfaces?

There are two primary ways to create an object shape in TypeScript: interface and type (Type Alias). Generally, standard practice dictates:

• Use interface whenever you are constructing objects, especially objects communicating with databases or APIs.
• Use type when generating highly specific logical subsets, Tuples or specific Union strings (type Size = "Small" | "Large").