TypeScript Interface vs Type Explained

Welcome back! I am Mihir, and in this lesson we will compare interface vs type in TypeScript.

Both can describe object shapes, but they are not exactly the same.

Similarity: Both Can Describe Objects

Using interface:

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

Using type:

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

For simple object shapes, both work well.

Interfaces Can Extend Interfaces

interface Person {
  name: string;
}

interface Employee extends Person {
  employeeId: number;
}

Employee includes all properties from Person.

Types Can Use Intersections

type Person = {
  name: string;
};

type Employee = Person & {
  employeeId: number;
};

This gives a similar result using &.

Types Can Represent Unions

This is where type is often better.

type Status = "pending" | "success" | "failed";

An interface cannot directly represent this kind of union.

You cannot write:

interface Status = "pending" | "success";

So for unions, use type.

Types Can Name Primitives

type UserId = number;
type Email = string;

Interfaces are for object-like structures, not primitive aliases.

Interfaces Can Merge

Interfaces with the same name are merged.

interface Config {
  theme: string;
}

interface Config {
  language: string;
}

const config: Config = {
  theme: "dark",
  language: "en",
};

This can be useful when extending library types.

Type Aliases Cannot Merge

type Config = {
  theme: string;
};

type Config = {
  language: string;
};

This causes an error because a type alias name can only be declared once in the same scope.

Which One Should You Use?

Use interface when:

• you are describing object shapes
• you expect extension
• you are working with class contracts
• you want declaration merging

Use type when:

• you need unions
• you need primitive aliases
• you need tuple aliases
• you are composing complex types

Practical Rule

For beginners, this rule works well:

// Object shape
interface User {
  id: number;
  name: string;
}

// Union or helper type
type Status = "loading" | "success" | "error";

This keeps your code predictable.

Quick Recap

• interface and type can both describe objects.
• type is better for unions, primitives, tuples, and complex compositions.
• interface supports declaration merging.
• There is no single winner; choose based on the job.

Next up, we will learn Extending Interfaces → in more detail.