TypeScript: What is the difference between type and interface?

If you’re venturing into TypeScript development, you’ve likely come across the terms “type” and “interface.” While they may seem similar at first glance, they have distinct differences and serve different purposes in TypeScript. Understanding these differences can help you make informed decisions when it comes to choosing between the two.

In this article, we’ll explore the disparities between types and interfaces in TypeScript, and provide examples and appropriate use cases for each. By the end, you’ll have a clear understanding of when to use types and interfaces and how they can enhance your TypeScript development workflow.

TypeScript Types

In TypeScript, the concept of “types” refers to defining the shape and behavior of data. Types allow you to define primitive types like strings, numbers, booleans, and more. They can also be used to create more complex types such as union types, intersection types, and tuple types.

Let’s consider a simple example of defining a type:

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

In this case, we’ve created a type called “Person” that describes the structure of a person object, including their name, age, and email. Types provide a convenient way to reuse and encapsulate complex type definitions.

One of the key advantages of types in TypeScript is their flexibility. They can be used for a wide range of use cases, including describing the shape of data, defining function signatures, providing type aliases, and more. Types also support various advanced features such as conditional types and mapped types, which can bring additional power and expressiveness to your code.

TypeScript Interfaces

Interfaces, on the other hand, define contracts that an object must adhere to. They specify the structure and behavior of an object, including its properties, methods, and events. Interfaces are primarily used for object-oriented programming and provide a way to establish a standard structure that classes can implement.

Here’s an example of an interface that describes a person object:

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

Similar to types, interfaces can be used to define complex object structures and enforce consistency across multiple objects. However, interfaces have a unique feature called “declaration merging” that allows multiple interface definitions with the same name to be automatically merged into a single interface definition. This can be beneficial when dealing with codebases that involve multiple contributors and modules.

Interfaces are particularly useful when working with object-oriented concepts like inheritance and polymorphism. They provide a clear contract that classes can implement, ensuring that the required properties and methods are present.

Differences Between Types and Interfaces

While types and interfaces have similar use cases, there are a few key differences that set them apart. Let’s explore these differences in more detail:

1. Flexibility:

Types offer more flexibility compared to interfaces. They can define not only the shape of objects but also alias primitive types, define union types, intersection types, and more. Types allow you to express a wide range of complex type definitions, making them suitable for various scenarios.

Interfaces, on the other hand, are more rigid in their structure. They are primarily used for describing the shape of objects and enforcing contracts. While interfaces don’t provide advanced features like union types or intersections, they excel in providing a clear structure for defining object-oriented concepts.

2. Declaration Merging:

One of the unique features of interfaces is declaration merging. When you declare multiple interfaces with the same name, TypeScript automatically merges them into a single interface definition. This can be useful in situations where you want to extend existing interfaces or combine multiple interfaces into one.

Types, however, do not support declaration merging. If you declare multiple types with the same name, TypeScript will throw an error. This limitation makes interfaces more suitable for scenarios where you need to extend or combine multiple interface definitions.

3. Implementing and Extending Classes:

Interfaces have direct support for implementing and extending classes. You can define an interface and have classes implement it by adhering to its contract. This allows you to define common behavior and ensure consistency across different classes.

Types, on the other hand, cannot be directly implemented or extended by classes. While types can be used to define the shape of data inside a class, they cannot enforce the class to adhere to a specific contract. If you need to define class behavior, interfaces are a better choice.

4. Additional Type Features:

Typescript’s type aliases provide additional features that are not available in interfaces. Types support conditional types, which allow you to create types that depend on conditionals or predicates. They also support mapped types, which enable you to transform existing types into new ones based on specific rules.

Interfaces, on the other hand, do not provide these advanced type features. If you find yourself needing to perform complex type transformations or create conditional types, types are the way to go.

When to Use Types vs Interfaces

Now that we’ve explored the differences between types and interfaces, you might be wondering when to use each. Here are some guidelines to help you make the right choice:

Use Types When:

  • You need advanced type features like conditional types and mapped types.
  • You want to define complex types or aliases for primitive types.
  • You need to define union types or intersection types.
  • You’re working with functions and need to define function signatures.

Use Interfaces When:

  • You’re following object-oriented programming principles and need to enforce contracts.
  • You need to extend or merge multiple interface definitions.
  • You want to define the structure and behavior of objects.
  • You’re working with classes and need to implement interfaces.

In general, types offer more expressive power and advanced type features, making them ideal for complex type definitions and scenarios where you need fine-grained control over types. Interfaces, on the other hand, are useful for enforcing contracts, implementing class behavior, and allowing declaration merging.

Ultimately, the choice between types and interfaces depends on your specific use case and personal preference. Both types and interfaces have their strengths and can be used together to create a powerful and flexible TypeScript codebase.

In conclusion, understanding the differences and use cases of types and interfaces in TypeScript is crucial for making informed decisions in your codebase. Types provide flexibility and advanced type features, while interfaces enforce contracts and allow for declaration merging. By understanding when to use each, you can write cleaner, more maintainable TypeScript code.

Remember, the choice between types and interfaces ultimately depends on your specific needs and coding style. Experiment with both, and let your codebase guide you in choosing the right tool for the job.

Keep coding, and embrace the power of TypeScript in your projects!