Types vs. Interfaces in TypeScript - Fraser Walker-Saunders
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Types vs. Interfaces in TypeScript

Fraser Walker-Saunders  at 

Types vs. Interfaces in TypeScript

TypeScript offers two fundamental mechanisms for defining types: types and interfaces. These constructs play a crucial role in shaping your data and ensuring type safety in your TypeScript codebase. In this article, we’ll explore the differences between types and interfaces and when to use each effectively.

Quick Overview

Before we delve into the details, let’s summarize some key differences between types and interfaces:

  • Use types by default: In most cases, it’s recommended to use types by default unless you specifically need a feature offered by interfaces, such as the extends keyword.

  • Interfaces for object inheritance: If you’re working with objects that inherit from each other, interfaces are generally preferable. They optimize type checking when using the extends keyword.

  • Types for unions and advanced types: Types are more versatile and can express unions, mapped types, and conditional types. Use types when you need these advanced features.

Now, let’s explore these concepts in more detail.

Interfaces: Object Inheritance and More

Object Inheritance

Interfaces excel when you need to define types that can be inherited from. For example, consider an interface for a User object with properties like name and id:

interface User {
  name: string;
  id: number;

You can extend this interface to create more specific types, such as Customer and Employee, inheriting properties from the User interface:

This inheritance mechanism allows you to build a hierarchy of types, promoting code reuse and organization. Index Signatures

Interfaces come with an implicit index signature. This enables you to use object spread syntax and destructuring more flexibly:

interface Point {
  x: number;
  y: number;

const point: Point = { x: 1, y: 2 };

const updatedPoint = { ...point, z: 3 }; // No error, thanks to the implicit index signature.

Types: Versatility and Advanced Features


Types offer versatility by allowing you to define a wide range of types, including unions, mapped types, and conditional types. These features enable you to express complex and flexible types:

type Status = "active" | "inactive";

type Account = {
  id: number;
  status: Status;

type OptionalProps<T> = {
  [K in keyof T]?: T[K];

With types, you can create types that adapt to various situations and requirements.

Choosing Between Interfaces and Types

In practice, the choice between interfaces and types depends on your specific use case:

  • Use Interfaces by Default: When dealing with object types that require inheritance or when you want to take advantage of the implicit index signature, interfaces are the preferred choice. They promote clean code organization and facilitate structured composition.

  • Use Types When Needed: If your type doesn’t need inheritance, or you require advanced type features like unions or mapped types, types are your go-to option. They offer the flexibility to craft complex types independently.

Here’s a concise summary of the key distinctions:

Implicit Index SignatureYesNo
Unions, Mapped Types, Conditional TypesLimitedYes


Types and interfaces are both powerful tools for defining types in TypeScript. The best choice for your project depends on your specific requirements. As a general guideline, start with interfaces for object types and use types when dealing with standalone types or advanced type features.

Understanding these distinctions enables you to make informed choices and write more robust TypeScript code. Happy typing!