Interface vs. Type in TypeScript: When to Use Which and Why

Summary

• Interfaces are ideal for defining object shapes and are more performant when extending structures, thanks to optimized extends.
• Types are flexible, allowing unions, intersections, and the aliasing of primitives and complex structures, but they don’t extend natively and lack declaration merging.
• For codebases that involve large object hierarchies, always consider interfaces with extends over type intersections (&) for optimized performance and maintainability.

In TypeScript, two powerful constructs, interfaces and types, allow developers to define the shape of data. They are similar, but there are nuanced differences in their usage and behavior, which makes understanding them essential for writing clean and efficient TypeScript code.

Let’s dive into the specifics of interfaces and types, examine their differences, and see why choosing the right one can impact performance and code readability.

Understanding Interfaces

In TypeScript, an interface defines the shape of an object. Interfaces are mainly used to describe object structures and define contracts within your code.

Key Features of Interfaces:

• Extendable: Interfaces can be extended, meaning they can inherit properties from other interfaces. This allows for modular and reusable code.
• Merging Capabilities: Interfaces support declaration merging, meaning if you define an interface multiple times, TypeScript will merge them into a single definition.
• Performance Optimization: TypeScript’s compiler is optimized for interfaces, particularly when extending or merging properties, making interfaces slightly more performant than type aliases.

Examples using interface

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

interface Employee extends User {
  position: string;
}

const employee: Employee = {
  id: 1,
  name: "Alice",
  position: "Developer",
};

        

In this example, Employee extends User, inheriting its properties (id and name). This is highly readable and clean, making interfaces a preferred option for object hierarchies.

Understanding Types

Type aliases provide a way to define a custom type in TypeScript. Unlike interfaces, types are more flexible and can be used for functions, primitive values, and even complex unions and intersections.

Key Features of Types:

• Union and Intersection Types: Types can combine multiple types using unions (|) and intersections (&). This is useful for complex type compositions.
• Aliases for Any Type: Types can alias primitives, unions, arrays, tuples, and more, making them highly versatile.
• Non-Extendable by Default: Unlike interfaces, types cannot be extended in a straightforward manner, making them better suited for single, non-hierarchical type definitions.

Example: Using Types

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

type Employee = User & {
  position: string;
};

const employee: Employee = {
  id: 1,
  name: "Alice",
  position: "Developer",
};
        

Here, we use a type alias User and combine it with additional properties for Employee using the intersection (&). While this approach works, it is generally recommended to use interface extends instead of type &.

Interface extends vs. Type Intersections (&)

While both interface extends and type intersections (&) allow combining types, there is a key performance difference between the two. TypeScript’s compiler is optimized to handle interfaces with extends more efficiently than type intersections. When combining multiple types using &, the compiler has to evaluate all properties across types, which can lead to increased complexity and potentially impact performance in large codebases.

Performance Comparison Example

Consider a scenario where we have two separate user types that we want to combine into an extended type or interface.

Using Interface extends

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

interface PremiumUser extends BasicUser {
  subscriptionType: string;
}

const premiumUser: PremiumUser = {
  id: 2,
  name: "John Doe",
  subscriptionType: "Gold",
};        

In this example, PremiumUser extends BasicUser, inheriting all of its properties. TypeScript compiles this combination more efficiently because it directly links the two interfaces.

Using Type Intersections (&)

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

type PremiumUser = BasicUser & {
  subscriptionType: string;
};

const premiumUser: PremiumUser = {
  id: 2,
  name: "John Doe",
  subscriptionType: "Gold",
};        

In this example, PremiumUser is created by intersecting BasicUser with additional properties. TypeScript must process each property of the intersected types, which can be less efficient in more complex or deeply nested structures.

Best Practice: Use Types by Default, but Prefer Interface extends Over &

• Use types by default for their versatility in representing different constructs.
• For defining object shapes that may need to extend other types, prefer interfaces with extends over type intersections (&).
• This approach maintains performance advantages while keeping code modular and readable.

Summary

• Interfaces are ideal for defining object shapes and are more performant when extending structures, thanks to optimized extends.
• Types are flexible, allowing unions, intersections, and the aliasing of primitives and complex structures, but they don’t extend natively and lack declaration merging.
• For codebases that involve large object hierarchies, always consider interfaces with extends over type intersections (&) for optimized performance and maintainability.