Interfaces vs Type Aliases: In-depth Comparison

Table of Contents

• Introduction
• What Are Interfaces in TypeScript?
  • Basic Syntax and Use Case
  • Interface Declaration Merging
• What Are Type Aliases in TypeScript?
  • Basic Syntax and Use Case
  • Flexible Types with Type Aliases
• Key Differences Between Interfaces and Type Aliases
  • Extending and Implementing
  • Declaration Merging
  • Flexibility and Use Cases
• Practical Examples and Use Cases
  • When to Use Interfaces
  • When to Use Type Aliases
• Conclusion

Introduction

When working with TypeScript, interfaces and type aliases are two powerful tools that allow you to define complex types for objects, arrays, functions, and more. While they might seem similar at first glance, there are subtle but important differences between them. Understanding when and how to use each one is crucial for writing clean, maintainable, and type-safe code.

In this article, we will provide an in-depth comparison of interfaces and type aliases in TypeScript. We’ll look at their syntax, capabilities, use cases, and practical examples to help you decide when to use one over the other.

What Are Interfaces in TypeScript?

An interface in TypeScript is a way to define a contract for the shape of an object. It describes the structure, including the properties and methods, that an object should have. Interfaces are especially useful in object-oriented programming for enforcing type safety and consistency.

Basic Syntax and Use Case

The syntax for defining an interface is straightforward. You use the interface keyword, followed by the interface name, and then define the object shape within curly braces.

interface Person {
  name: string;
  age: number;
  greet(): void;
}

In this example, the Person interface defines an object with name (string), age (number), and a greet() method.

Interfaces are typically used to define object shapes, especially when you want to enforce that certain classes or objects adhere to a specific structure.

Interface Declaration Merging

One of the most powerful features of interfaces is declaration merging. This allows multiple declarations of the same interface to be automatically combined into a single definition.

interface Person {
  name: string;
}

interface Person {
  age: number;
}

const person: Person = {
  name: "John",
  age: 30
};

In the above example, the Person interface is declared twice, and TypeScript merges them into a single interface with both name and age properties.

What Are Type Aliases in TypeScript?

A type alias allows you to define a type with a specific name. Unlike interfaces, type aliases can represent more than just object shapes—they can represent primitives, unions, intersections, tuples, and more.

Basic Syntax and Use Case

You define a type alias using the type keyword. Type aliases can be used to define object types, as well as more complex types like union and intersection types.

type Person = {
  name: string;
  age: number;
  greet(): void;
};

In this example, the Person type alias defines the same structure as the previous interface, but we are using a type instead of an interface.

Flexible Types with Type Aliases

Type aliases offer greater flexibility compared to interfaces because they can represent a broader range of types. They allow you to define union and intersection types, which are not possible with interfaces alone.

type ID = string | number;

In this case, the ID type alias can represent either a string or a number.

You can also use type aliases for more complex scenarios like tuples or function types:

type Point = [number, number];
type Callback = (message: string) => void;

Key Differences Between Interfaces and Type Aliases

Both interfaces and type aliases allow you to define object shapes and provide type safety, but there are key differences in how they behave.

Extending and Implementing

• Interfaces can extend other interfaces using the extends keyword, which allows you to build on top of existing interfaces.

interface Animal {
  name: string;
}

interface Dog extends Animal {
  breed: string;
}

const dog: Dog = {
  name: "Rex",
  breed: "Golden Retriever"
};

• Type aliases, on the other hand, use intersection types (&) to extend types.

type Animal = {
  name: string;
};

type Dog = Animal & {
  breed: string;
};

const dog: Dog = {
  name: "Rex",
  breed: "Golden Retriever"
};

Both approaches achieve the same result, but interfaces have a more natural syntax for extending, making them the preferred choice when working with OOP and class-based systems.

Declaration Merging

• Interfaces support declaration merging, allowing you to define the same interface multiple times in different parts of your code, and TypeScript will automatically combine them.

interface Person {
  name: string;
}

interface Person {
  age: number;
}

const person: Person = {
  name: "John",
  age: 30
};

• Type aliases do not support declaration merging. If you try to define a type alias with the same name multiple times, TypeScript will throw an error.

type Person = {
  name: string;
};

type Person = {  // Error: Duplicate identifier 'Person'.
  age: number;
};

Flexibility and Use Cases

• Interfaces are most commonly used for defining object shapes and classes. They are more suited for scenarios where you want to define a contract for an object or class and use inheritance or implementation.
• Type aliases are more flexible, allowing you to define not only objects but also unions, intersections, tuples, and more. They are ideal for situations where you need to represent more complex types, like a combination of different types or function signatures.

Practical Examples and Use Cases

When to Use Interfaces

• Class-based systems: Interfaces are perfect when working with object-oriented programming (OOP), where you need to define the structure for classes and ensure that they adhere to a contract. interface Animal { name: string; speak(): void; } class Dog implements Animal { name: string; constructor(name: string) { this.name = name; } speak() { console.log(`${this.name} says woof!`); } }
• Data modeling: Use interfaces when you need to model data that represents objects with well-defined structures (e.g., user objects, product data). interface Product { id: number; name: string; price: number; }

When to Use Type Aliases

• Unions and intersections: Type aliases shine when you need to define more complex types like unions or intersections. type Shape = Circle | Rectangle; type Circle = { kind: "circle"; radius: number }; type Rectangle = { kind: "rectangle"; width: number; height: number };
• Function signatures and tuples: Type aliases can define function signatures, tuples, and other more advanced types that interfaces cannot. type Operation = (a: number, b: number) => number; const add: Operation = (a, b) => a + b;

Conclusion

While both interfaces and type aliases provide robust ways to define types in TypeScript, they serve different purposes. Interfaces are best suited for defining object shapes, ensuring that classes adhere to specific contracts, and leveraging inheritance. On the other hand, type aliases offer more flexibility, allowing you to define unions, intersections, tuples, and other complex types.

• Use interfaces when you need to define object structures and work with class-based systems or need the benefit of declaration merging.
• Use type aliases when you need to define more complex types, such as unions, intersections, tuples, or function signatures.

Both tools are invaluable in TypeScript, and understanding when to use each will help you write more maintainable, scalable, and type-safe code.