Apprendre Interfaces vs Type Aliases | Understanding Interfaces

When working in TypeScript, you have two main ways to describe the shape of objects: interfaces and type aliases. While both can be used to define the structure of data, there are important differences that influence which one you should choose for a given situation. A type alias uses the type keyword and can represent not only object shapes, but also primitives, unions, intersections, and more. In contrast, an interface is specifically designed to describe the structure of objects and is more focused on object-oriented patterns.


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// Using an interface to define a shape
interface User {
  id: number;
  name: string;
  email: string;
}

// Using a type alias to define the same shape
type UserType = {
  id: number;
  name: string;
  email: string;
};

// Example usage
const alice: User = { id: 1, name: "Alice", email: "alice@example.com" };
const bob: UserType = { id: 2, name: "Bob", email: "bob@example.com" };

The main differences between interfaces and type aliases become clear as your codebase grows:

Extension: interfaces can be extended using the extends keyword, allowing you to build up complex types from simpler ones;
Merging: interfaces support declaration merging—you can declare the same interface multiple times and TypeScript will combine their properties. This is useful when working with third-party libraries or large codebases;
Use cases: type aliases are more flexible in the types they can represent, including unions, intersections, and primitives—not just objects. They do not support declaration merging, and while you can combine types using intersections (&), this approach lacks some of the expressive power and clarity of interface extension.

Note

Choose interfaces to describe the shape of objects, especially if you anticipate needing to extend or merge them. Use type aliases to represent more complex or varied types, such as unions or intersections, or when working with primitives alongside object shapes.

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Section 1. Chapitre 4

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              1234567891011121314151617
            
// Using an interface to define a shape
interface User {
  id: number;
  name: string;
  email: string;
}

// Using a type alias to define the same shape
type UserType = {
  id: number;
  name: string;
  email: string;
};

// Example usage
const alice: User = { id: 1, name: "Alice", email: "alice@example.com" };
const bob: UserType = { id: 2, name: "Bob", email: "bob@example.com" };

The main differences between interfaces and type aliases become clear as your codebase grows:

Extension: interfaces can be extended using the extends keyword, allowing you to build up complex types from simpler ones;
Merging: interfaces support declaration merging—you can declare the same interface multiple times and TypeScript will combine their properties. This is useful when working with third-party libraries or large codebases;
Use cases: type aliases are more flexible in the types they can represent, including unions, intersections, and primitives—not just objects. They do not support declaration merging, and while you can combine types using intersections (&), this approach lacks some of the expressive power and clarity of interface extension.

Note

Tout était clair ?

Merci pour vos commentaires !

Section 1. Chapitre 4