Alejandro Galaz

When developers dive into TypeScript, one of the first choices they face is whether to use types or interfaces. These two features offer powerful ways to define and enforce data structures, but which one should you use? The answer depends on your project size, use cases, and even the preferences of your development team. In this article, we’ll break down the key differences between types and interfaces, explore their use cases, and share best practices for using each in modern TypeScript development.

1. What Are Types and Interfaces?

Before we dive into their differences, let’s start with a quick overview of what types and interfaces are in TypeScript.

Types

• Definition: A type in TypeScript is a way to define the structure of a variable, function, or object.
• Usage: Types allow you to describe the shape of data, whether it’s a primitive, array, object, or function signature.

type User = {
  id: number;
  name: string;
  age?: number;
};

Interfaces

• Definition: An interface defines the contract for the structure of an object, specifying its properties and their types.
• Usage: Interfaces are more commonly used for defining object structures and can be extended or implemented by classes.

interface User {
  id: number;
  name: string;
  age?: number;
}

Both types and interfaces help TypeScript developers create well-structured, readable code by enforcing consistent data structures across their applications. However, they each have their strengths and weaknesses. Let’s explore these in detail.

2. Key Differences Between Types and Interfaces

While types and interfaces can sometimes seem interchangeable, there are several key differences between them. Understanding these differences can help you decide when to use one over the other.

1. Declaration Merging

• Interfaces: One of the most significant features of interfaces is declaration merging. This means that if you declare an interface more than once, TypeScript will merge the properties from all declarations.

interface User {
  id: number;
  name: string;
}

interface User {
  age?: number;
}

const user: User = { id: 1, name: "John", age: 25 }; // Valid

• Types: Types do not support declaration merging. Once a type is defined, it cannot be modified or merged with other types.

type User = {
  id: number;
  name: string;
};

type User = {
  age?: number; // Error: Duplicate identifier 'User'
};

2. Union and Intersection Types

• Types: Types shine when it comes to union and intersection types, offering greater flexibility for complex data structures.
• Union Types allow a variable to have one of several types:

type ID = number | string;

• Intersection Types combine multiple types into one:

type Address = { street: string; city: string };
type Contact = { phone: string };
type UserDetails = Address & Contact;

• Interfaces: Interfaces do not support union types directly. While you can use intersection types with interfaces, the syntax can become more cumbersome.

3. Extensibility

• Interfaces: Interfaces are designed to be extended, making them ideal for large applications where data models evolve over time.

interface User {
  id: number;
  name: string;
}

interface Admin extends User {
  role: string;
}

const admin: Admin = { id: 1, name: "Alice", role: "Manager" };

• Types: Types can also be extended, but it’s less intuitive than with interfaces.

type User = {
  id: number;
  name: string;
};

type Admin = User & {
  role: string;
};

4. When to Use Types vs Interfaces

• For simpler, smaller-scale projects: Types offer more flexibility with union and intersection types, making them a better choice for individual components or isolated parts of the codebase.
• For large-scale, object-oriented projects: Interfaces are generally preferred due to their support for declaration merging and easier extensibility.

3. Types vs Interfaces: Real-World Use Cases

Let’s look at some real-world examples to better understand how to use types and interfaces in different scenarios.

1. Defining Object Shapes

Both types and interfaces can define the shape of an object. However, interfaces are often preferred for defining large, complex objects, especially when you anticipate the object’s structure will change over time.

interface User {
  id: number;
  name: string;
  email: string;
}

type Address = {
  street: string;
  city: string;
};

2. Extending and Merging

Interfaces allow for easy extension and declaration merging, which makes them particularly useful when working with evolving data models, such as when working with APIs.

interface APIResponse {
  status: number;
  message: string;
}

interface UserResponse extends APIResponse {
  user: User;
}

3. Union and Intersection Types

Types excel in scenarios where you need to use union or intersection types, such as combining multiple types into one or allowing a variable to be one of several types.

type Shape = Circle | Square;

type Circle = {
  radius: number;
};

type Square = {
  sideLength: number;
};

4. How Types and Interfaces Affect Code Readability

Choosing between types and interfaces can significantly affect the readability of your code, especially in large projects or team environments.

1. Code Clarity

• Interfaces: When working on large projects, interfaces can make code more readable by providing clear, extendable structures. Interfaces are often easier to understand at a glance because they are more declarative and self-explanatory.
• Types: Types are flexible and concise, but in some cases, they can make the code harder to understand, especially when union or intersection types are heavily used.

2. Maintenance

• Interfaces: Interfaces are ideal for codebases that require scalability. Declaration merging allows multiple developers to extend interfaces without introducing conflicts, making it easier to maintain code over time.
• Types: Types are better for isolated parts of your code that don’t require extension or modification.

// Interface Example
interface Vehicle {
  wheels: number;
  brand: string;
}

interface Car extends Vehicle {
  doors: number;
}

// Type Example
type Vehicle = {
  wheels: number;
  brand: string;
};

type Car = Vehicle & {
  doors: number;
};

5. Best Practices for Using Types vs Interfaces in Large-Scale Projects

Here are some best practices to keep in mind when deciding between types and interfaces in large-scale TypeScript projects.

1. Consistency

Choose one approach and stick with it across your project. Mixing types and interfaces can lead to confusion, especially when collaborating with other developers.

2. Use Interfaces for Objects

Interfaces are generally the better choice for defining object structures, especially when you anticipate extending them.

3. Use Types for Flexibility

Types should be your go-to for flexible and concise data models, particularly when working with union and intersection types.

4. Consider Extensibility

If you know that a data model will need to be extended in the future, use an interface. If not, a type might be a better choice for its simplicity.

Conclusion

When deciding between types and interfaces in TypeScript, consider the complexity, size, and long-term needs of your project. While types are more flexible and better suited for smaller components or utility types, interfaces shine in large-scale projects that require extensibility and readability. By understanding the strengths and limitations of each, you can make informed decisions and write more maintainable, scalable TypeScript code.