TypeScript Interfaces Explained

Welcome back! I am Mihir, and in this lesson we will learn interfaces in TypeScript.

An interface describes the shape of an object.

Basic Interface

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

const user: User = {
  id: 1,
  name: "Mihir",
  email: "mihir@example.com",
};

The object must match the interface structure.

Missing Properties

Invalid:

const user: User = {
  id: 1,
  name: "Mihir",
};

email is required, so TypeScript shows an error.

Optional Properties

Use ? to make a property optional.

interface Product {
  id: number;
  name: string;
  description?: string;
}

Valid:

const product: Product = {
  id: 101,
  name: "Mouse",
};

Readonly Properties

interface Order {
  readonly id: number;
  total: number;
}

const order: Order = {
  id: 5001,
  total: 999,
};

You can update total.

order.total = 1099;

But you cannot update id.

order.id = 5002;

Interfaces with Methods

Interfaces can describe methods too.

interface Logger {
  log(message: string): void;
}

const consoleLogger: Logger = {
  log(message) {
    console.log(message);
  },
};

The object must provide a log method that accepts a string and returns nothing.

Function Properties in Interfaces

You can also write methods as function properties.

interface Calculator {
  add: (a: number, b: number) => number;
}

const calculator: Calculator = {
  add: (a, b) => a + b,
};

Both styles are common.

Extending Interfaces

An interface can build on another interface.

interface Person {
  name: string;
}

interface Employee extends Person {
  employeeId: number;
}

const employee: Employee = {
  name: "Mihir",
  employeeId: 101,
};

Employee has both name and employeeId.

Declaration Merging

Interfaces with the same name can merge.

interface Settings {
  theme: string;
}

interface Settings {
  language: string;
}

const settings: Settings = {
  theme: "dark",
  language: "en",
};

This is called declaration merging.

Quick Recap

• Interfaces describe object shapes.
• They support required, optional, and readonly properties.
• They can describe methods and function properties.
• Interfaces can extend other interfaces.
• Interfaces with the same name can merge.

Next up, we compare Interface vs Type → so you know which one to choose in real projects.