TypeScript Type Aliases Explained

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

A type alias gives a name to a type so you can reuse it across your code.

Basic Type Alias

type Username = string;

let name: Username = "Mihir";

Username is now another name for string.

This is useful when a basic type has business meaning.

Object Type Alias

The most common use is naming object shapes.

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

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

Now you can use User anywhere you need this structure.

Optional Properties

type Product = {
  id: number;
  name: string;
  description?: string;
};

description is optional.

Valid:

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

Readonly Properties

type Account = {
  readonly id: number;
  email: string;
};

You can read id, but you cannot change it after creation.

const account: Account = {
  id: 1,
  email: "user@example.com",
};

account.email = "new@example.com";

Invalid:

account.id = 2;

Union Type Alias

Type aliases are excellent for unions.

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

let paymentStatus: Status = "pending";

Invalid:

paymentStatus = "done";

Only the listed values are allowed.

Function Type Alias

You can also name function shapes.

type CalculateTotal = (price: number, quantity: number) => number;

const calculateTotal: CalculateTotal = (price, quantity) => {
  return price * quantity;
};

This keeps callback-heavy code much cleaner.

Combining Types with Intersection

type HasId = {
  id: number;
};

type HasTimestamps = {
  createdAt: Date;
  updatedAt: Date;
};

type DatabaseUser = HasId & HasTimestamps & {
  name: string;
};

DatabaseUser must have all properties from all combined types.

Type Alias Cannot Be Reopened

You cannot define the same type alias twice.

type User = {
  name: string;
};

type User = {
  email: string;
};

This causes an error.

Interfaces behave differently, which we will learn next.

Quick Recap

• A type alias gives a reusable name to a type.
• Use type for objects, unions, literals, functions, and intersections.
• Type aliases make code easier to read and maintain.
• A type alias cannot be reopened with another declaration of the same name.

Next up, we will learn Interfaces →, another powerful way to describe object shapes in TypeScript.