Types vs. Interfaces in TypeScript

One of the key debates within the TypeScript community revolves around whether to use types or interfaces when defining structures in your code. While the TypeScript documentation recommends using interfaces unless specific features of types are needed, some developers argue that types are a superior choice in most cases. In this article, we’ll explore the reasons behind this perspective and examine scenarios where interfaces might have an edge.

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Table of contents

Versatility of Types

The primary argument in favor of using types is their versatility. Types in TypeScript offer more ways to define and use them compared to interfaces. Types can represent not only object structures but also unions of different types, providing a more flexible and expressive way to model data.

// Example of a type defining a union
type SomeType = string | number;

While interfaces are limited to defining the structure of an object, types allow developers to handle a broader range of scenarios, making them a more powerful tool in many situations.

Unions and Intersection

Types support creating unions and intersections effortlessly, allowing developers to compose complex types by combining multiple simpler ones. This capability is crucial for handling various data shapes in a concise manner.

// Example of a type with union and intersection
type CombinedType = { name: string } & { age: number };

While interfaces can achieve a form of extension through the extends keyword, they lack the concise syntax for creating unions directly. This makes types more attractive for scenarios where you need to represent complex data structures.

Consistency in Codebase

Using types consistently throughout a codebase can lead to better readability and maintainability. When developers use interfaces for some structures and types for others, it can introduce unnecessary complexity. Types provide a consistent approach, making it easier for developers to understand and work with the code.

Interface Myth: Speed

There’s a common misconception that interfaces are faster than types during the compilation of TypeScript code. However, the performance difference between interfaces and types is negligible in modern TypeScript versions. Choosing between types and interfaces based on speed alone is not a valid criterion.

Interface Benefit: Merging

One notable advantage of interfaces is their ability to merge together. This allows developers to extend existing interfaces, a feature that types lack. While merging interfaces can be powerful for extending external libraries or global objects, it comes with the risk of unintentionally combining different structures, potentially leading to issues.

// Example of merging interfaces
interface Person {
  name: string;
}
interface Person {
  age: number;
}

const myUser: Person = { name: "Bod", age: 40 };

In scenarios where merging interfaces is necessary, interfaces might be the preferred choice.

Conclusion

The debate between types and interfaces in TypeScript often boils down to personal preference and the specific needs of a project. While types offer more versatility and consistency, interfaces provide a unique merging capability. Understanding the strengths and weaknesses of both can help you make informed decisions based on your project’s requirements.

In practice, many developers find that using types as the default choice leads to cleaner and more maintainable code. However, it’s essential to be aware of situations where interfaces might be more suitable, such as when extending external definitions or merging interfaces for specific use cases. Ultimately, the key is to strike a balance and choose the approach that best fits the needs of your project and team.