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The Difference Between TypeScript Interface vs Type – Unlocking Code Clarity

Q: What is a type alias in TypeScript?

A type alias in TypeScript allows developers to create a new name for a type. It helps simplify complex type definitions and improve code readability.

Q: Can I use both type aliases and interfaces in the same project?

Yes, they serve different purposes and can coexist harmoniously in your TypeScript code. Choose the tool that best suits the specific use case for optimal type safety and maintainability.

Q: Can Type Aliases and Interfaces be used in generic types?

Yes, both Type Aliases and Interfaces can be used in generic types. This allows for creating reusable and flexible type definitions that can work with different data types and structures.

Last Updated on October 24, 2024

Quick Summary

This blog post explores the detailed comparison between TypeScript Interface vs Type on various parameters. Also, we will take a deeper look at the specific use cases and other factors that differentiate TypeScript Type vs Interface to help you gain more insight and decide which is better for defining the object’s structure in TypeScript.

Introduction

On a circumferential level, comparing Type vs Interface TypeScript shares a few similar functionalities in defining the structure of the objects but diving deeper into the matrix, we can see that both possess a few key differences. An interface is ideal for declaring object shapes. It can be extended or merged, making it highly flexible when defining class contracts or reusable object patterns. At the same time, type is more versatile, allowing union, intersection, and other advanced type manipulations. While interfaces are generally preferred for defining models or APIs, types are chosen for more complex types and combinations.

Both TypeScript Interface vs Type serve a similar purpose, but the choice depends on the complexity and the flexibility required within the code.

TypeScript Interface vs Type - Comparison Table

TypeScript Type vs Interface showcases multiple differences on different levels and parameters, which helps us guide your decision based on the specific requirements of your projects. Let us take a glimpse of the tabular comparison between the two.

Feature	TypeScript Interface	TypeScript Type
Purpose	Defines the shape of an object or class	Defines the shape of objects, primitives, unions, intersections, and more
Extensibility	Other interfaces or types can extend it	Can be extended (intersected) using ‘&’
Merging	Supports declaration merging (can add fields across multiple declarations)	Does not support declaration merging
Type Composition	Extend interfaces or other types	Can compose complex types through unions (‘
Primitive Types	It cannot represent primitive types like string or number	It can represent primitive types like string, number, boolean, etc
Callable/Constructible Types	Supports defining callable and constructable types	Supports defining callable and constructable types
Declaration Syntax	Uses *interface* keyword	Uses *type* keyword
React Props & State	Preferred for defining component props and state in React	Can also define component props and state but often less preferred
Use Cases	Best for defining the structure of objects or classes in large codebases	Best for union, intersections, and complex type compositions

TypeScript Interface vs Type - Basic Syntax And Extending Types

In TypeScript, both Interfaces and Types have distinct syntax for defining and extending structures. Interfaces can extend other interfaces, while Types uses intersections (&) to combine multiple types, offering flexibility in creating complex definitions. Let us now understand the two in detail.

Basic Syntax - Type Aliases

The Type Aliases are created using the type keyword in TypeScript. They enable your development teams to define custom types by combining the existing types or creating new ones. The syntax for declaring a Type Alias involves mentioning the Alias name and the Type it represents. Generally, it is common to use capital letters for the first letter of the Type Alias name. To make it concise, the developers often use single uppercase letters, such as ‘T’ for generics.

Example – Type Alias

type TUserProps = {
  name: string;
  age: number;
};

Basic Syntax - Interfaces

Interfaces are defined using the keyword interface in TypeScript. They offer a way to declare custom types precisely for object shapes and contracts. The syntax here involves specifying the interface name and defining the structure of the type. Within Interfaces, it is common to use a capital letter for the first letter of the Interface name. Also, developers often use the ‘I’ as a prefix in the context of class names or object-like structures.

Example – Interface

interface IUserProps {
  name: string;
  age: number;
}

Extending Types - Type Aliases

Type Aliases use the (&) (ampersand) operator to combine multiple existing types, creating a new type. Referring to the example below, the ‘BusProps’ type is created by intersecting ‘VehicleProps’ and an additional type ‘company’ using ‘&’. This resulted in ‘BusProps’ including all properties from ‘VehicleProps’ and ‘company’.

Example – Type Alias

type VehicleProps = {
  name: string;
  registrationYear: number;
};
type BusProps = VehicleProps & {
  company: string;
};

Extending Types - Interfaces

The interfaces use the ‘extends’ keyword to declare extension or inheritance, allowing the creation of new interfaces that inherit properties from existing ones. Referring to the example below, the BusProps interface is declared by extending both the VehicleProps and the extra type ‘company’ using the extends keyword. The BusProps interface inherits properties from both the VehicleProps and ‘company’.

Example – Interface

interface VehicleProps {
  name: string;
  registrationYear: number;
}
interface BusProps extends VehicleProps {
  company: string;
}

Describing Objects and Base Types - Type Aliases vs Interfaces

Comparing TypeScript Interface vs Type, the Type Aliases can help describe a wide range of types, including primitive values (string, number, boolean), allowing for versatility in type definitions.

Example – Describing Primitive Values Using Type Alias

type Name = string;
const name: Name = "Bacancy";

Interfaces, however, are primarily designed to describe object shapes and are limited to doing so. Therefore, they cannot directly describe the primitive types.

Example – Interface Can Only Describe Objects

interface Name {
    name: string;
}
const address: Name ={
    name: "Bacancy"
}

Consistency and Readability - Interface vs Type TypeScript

The Type Aliases offer a consistent way to define the types with a simple syntax using the type keyword. This clean and concise syntax improves code readability, making it easier for your development teams to understand and maintain.

Example – Type Alias

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

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

  type EmployeeWithAddress = Employee & Address;

  // Usage
  const employee: EmployeeWithAddress = {
    id: 1,
    name: "Bacancy",
    city: "San Diego",
    zipCode: "91945",
  };

Now, comparing TypeScript Type vs Interface, merging interfaces with the same name can lead to unexpected behavior as the properties are combined. The syntax here for the merged or redeclared interfaces sometimes must be clarified precisely when dealing with larger codebases.

Example – Interfaces

  interface Employee {
    id: number;
    name: string;
  }
  // interface can be redeclared with the same name 
  interface Employee {
    city: string;
    zipCode: string;
  }

  interface EmployeeWithAddress extends Employee {}

  // Usage
  const employee: EmployeeWithAddress = {
    id: 1,
    name: "Bacancy",
    city: "New York",
    zipCode: "10005",
  };

Type Extraction - TypeScript Type vs Interface

Regarding Type Extraction between TypeScript Type vs Interface, the Type Aliases offer a concise and expressive syntax using the typeof operator to easily extract types from the existing objects.

Example – Type Alias

const product = {
    name: "Laptop",
    details: {
      brand: "Bacancy",
      price: 10000,
    },
    inStock: true,
  };

  // Type Alias using typeof
  type ProductDetails = typeof product.details;

  // Usage
  const laptopDetails: ProductDetails = {
    brand: "Bacancy",
    price: 10000,
  };
  // laptopDetails.color = "Silver"; // Error: Property 'color' does not exist on type 'ProductDetails'.

Interfaces, on the other hand, can quickly achieve similar results, but the syntax within Interface is less concise than Type Aliases as it involves directly specifying the extracted type.

Example – Interface

// Original Object
const project = {
  title: "Project 1",
  specification: {
    areaSize: 100,
    rooms: 3,
  },
};

// Interface achieving similar results
interface ProjectSpecs {
  areaSize: number;
  rooms: number;
}

// Usage
const specs: ProjectSpecs = {
  areaSize: 100,
  rooms: 3,
};

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Utility Types - TypeScript Type vs Interface

Within the TypeScript Interface vs Type, when using utility types like Omit, the type aliases offer a clean and direct syntax, making the code more readable and expressive.

Example – Type Alias

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

  // Utility Type: Omit
  type UserWithoutId = Omit;

  // Usage
  const userWithoutId: UserWithoutId = {
    name: "Bacancy",
    age: 23,
  };
  // userWithoutId.id = 123; // Error: Property 'id' does not exist on type 'UserWithoutId'.

Within TypeScript Interface vs Type, the Interface can use the utility types, but it typically requires the extends keyword, resulting in a slightly less elegant syntax than Type Aliases.

Example – Interface

interface Product {
    id: number;
    name: string;
    price: number;
  }

  // Utility Type: Omit
  interface DiscountedProduct extends Omit {
    discountPercentage: number;
  }

  // Usage
  const discountedItem: DiscountedProduct = {
    name: "Bacancy",
    price: 1000,
    discountPercentage: 10,
  };
  // discountedItem.id = 123; // Error: Property 'id' does not exist on type 'DiscountedProduct'.

Tuples - Typescript Interface vs Type

Regarding describing Tuples, comparing the TypeScript Interface vs Type, the Type Aliases offer a simple and cleaner syntax that helps describe tuples using the square brackets to represent the tuple structure. Considering the example below, the Address type is a Type Alias representing a tuple with elements of types number and string. The syntax within the Type Aliases is concise, using the square bracket to denote the tuple structure.

Example – Type Alias

type AddressTuple = [number, string, string];

// Utility Type: Omit
type AddressWithoutZip = Omit;

// Usage
const addressWithoutZip: AddressWithoutZip = ["City", "Street"];
// addressWithoutZip[0] = 123; // Error: Type '123' is not assignable to type 'undefined'.

Conversely, the interface can describe tuples but needs a more complex syntax involving read-only properties and explicitly specifying the tuple length. In the example below, the Address interface describes a tuple. The read-only and length properties are employed to ensure the tuple structure. The syntax here is more complex and requires explicitly specifying the index positions.

Example – Interface

interface Address {
  readonly 0: number;
  readonly 1: string;
  length: 2;
}

// Using the tuple interface
const homeAddress: Address = [123, "Main Street"];

Type Alias in Classes

The Type Aliases are well-suited for defining types related to class instances. They offer a clear, concise way to represent and interact with class objects. They are also flexible and consistent when working with various classes or when additional properties must be added to the class interfaces.

Example – Type Aliases in Classes

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

class Employee implements TEmployee {
  name: string;
  id: number;
  constructor(name: string, id: number) {
    this.name = name;
    this.id = id;
  }
}

Type vs Interface Typescript – When to Use Type Aliases

Type Aliases offer distinct advantages over Interfaces, making them an ideal choice in specific scenarios. Let’s explore some key areas where TypeScript Types stand out:

Describing Primitive Types: Type Aliases are highly versatile and can represent objects and primitive values like strings, numbers, and booleans. In contrast, Interfaces are primarily designed to shape objects. So, if you’re dealing with objects and primitive types, Type Aliases provide a more comprehensive solution.
Streamlined Use of Utility Types: Type Aliases integrate smoothly with utility types like Omit, resulting in cleaner and more readable syntax. This makes Type Aliases a better choice when manipulating types in your code, offering a more streamlined experience over Interfaces in TypeScript.
Consistent Syntax: Using an equal sign (=) for declaration, Type Aliases maintain a consistent syntax similar to other JavaScript declarations. This familiarity enhances readability, especially for developers well-versed in JavaScript. A consistent codebase is easier to understand and maintain, significantly benefiting scaling projects.
Predictable Behavior With Merging: Unlike Interfaces, Type Aliases are closed and cannot be merged or redeclared unintentionally. This helps prevent unexpected behavior in larger codebases, reducing the likelihood of bugs and inconsistencies. When predictability is key, Type Aliases offer a reliable solution.

Type vs Interface Typescript – When to Use Interfaces

On the other hand, Interfaces provides several unique benefits, particularly suited for specific use cases. Here’s why you might choose Interfaces over Types in TypeScript:

More Precise Error Messages: Interfaces often produce more concise and targeted error messages. Since Interfaces define specific contracts, violations trigger more focused error notifications, which can benefit large applications where clarity is crucial.
Ideal for Specific Use Cases: Interfaces are a better choice if you need shorter, more targeted error messages or have multiple implementations adhering to the same contract. This ensures that each implementation behaves consistently.
Merging Capabilities: One of the standout features of Interfaces is their ability to merge. You can extend an Interface across different parts of your code, and all extensions will be combined into a single definition. This feature is precious when building libraries or adding to definitions dynamically, offering flexibility that Types don’t provide.
Performance Considerations: Historically, performance was a consideration when choosing between TypeScript Types vs Interfaces.

However, recent observations suggest no significant performance difference between the two, meaning performance is no longer a deciding factor in modern TypeScript development.

Conclusion

When it comes to choosing between TypeScript Interface vs Type, the decision ultimately depends on a project’s specific requirements, and coding preferences. Both options offer strong advantages for writing a clean, maintainable code. By understanding their strengths, you can create a wel-structured TypeScript solutions that suits your needs. However, if you ar a business owner uncertain about which approach is best for your project, it is a good idea to hire dedicated developers. Their expertise will ensure you make the right choice and that your development aligns with your business goals efficiently.

Frequently Asked Questions (FAQs)

What is a type alias in TypeScript?

A type alias in TypeScript creates a custom name for an existing type, making complex types easier to manage and understand. It’s particularly useful for simplifying unions, intersections, or functions, improving code readability and maintainability without altering the underlying type.

Can I use both type aliases and interfaces in the same project?

Yes, TypeScript allows using both type aliases and interfaces in the same project. They complement each other, serving different purposes. Type aliases handle complex types, while interfaces define object shapes. Use them based on your project’s needs for better type safety.

Can Type Aliases and Interfaces be used in generic types?

Yes, both type aliases and interfaces support generics, allowing you to create flexible, reusable type definitions. This enables you to work with varying data types and structures while maintaining strong typing, making your TypeScript code more adaptable and maintainable.