▷ Interfaces vs Types in TypeScript: When to use each one?

TypeScript offers two main ways to define custom types: interface and type. Although in many cases they seem interchangeable, there are important differences that every developer should know to make informed decisions.

When to use Interfaces and when to use Types?

The choice between interface and type mainly depends on the specific use case:

• Interfaces: Ideal for defining the shape of objects and are extensible through declaration.
• Types: Perfect for unions, tuples, and more complex compositions.

Practical example: Key differences

Inheritance with Interfaces (extends)

interface Person {
  name: string;
  age: number;
}

interface Employee extends Person {
  position: string;
  salary: number;
}

Intersection with Types (&)

type Person = {
  name: string;
  age: number;
};

type Employee = Person & {
  position: string;
  salary: number;
};

The main difference is that interfaces allow declaration merging, which means you can extend an interface multiple times and TypeScript will automatically combine them.

Specific use cases

Interfaces for extensible objects

Interfaces are ideal when working with objects that can evolve, especially in libraries or APIs:

// You can extend the interface in different parts of the code
interface User {
  id: number;
  name: string;
}

// In another file
interface User {
  email: string;
}

// TypeScript automatically combines both declarations

Types for unions or complex compositions

Types are more flexible for creating unions, tuples, and conditional types:

// Union of types
type State = 'active' | 'inactive' | 'pending';

// Tuple
type Coordinates = [number, number];

// Conditional type
type NameOrAge<T> = T extends 'user' ? string : number;

Tips: Advantages of interfaces in IDE performance

Interfaces offer better performance in IDEs because:

• TypeScript can display the properties of interfaces more efficiently
• They are easier to inspect in development tools
• They provide better error messages
• They are more performant in very large projects

Conclusion

As a general rule, follow these recommendations:

• Use interfaces to define object shapes, especially in public APIs or libraries
• Use types for unions, tuples, or complex compositions
• In case of doubt, interfaces are usually the most recommended option for objects
• Maintain consistency in your project: choose an approach and stick to it