Programming Fundamentals › Type Systems

Type Alias

A new name for an existing type.

Also known as: typedef, type alias TypeScript

A type alias gives a new name to an existing type. It doesn’t create a new kind of value. It makes complex types shorter and more meaningful.

type UserId = string;
type Point = { x: number; y: number };
type Status = "pending" | "paid" | "shipped";
type Callback = (error: Error | null, result?: string) => void;

function distance(a: Point, b: Point): number { ... }

UserId = str
Point = tuple[float, float]       # Python: assignment, or `type Point = ...` in 3.12+
Handler = Callable[[Request], Response]

typedef unsigned long size_t;     // C's typedef

Why use them

• Readability: Point says more than { x: number; y: number }.
• One place to change when the underlying type changes.
• Documenting intent: UserId and OrderId tell readers what a string is for.
• Shortening long types, like function signatures or unions.

What they don’t do

An alias is interchangeable with the original. UserId and OrderId are both just string, so passing the wrong one isn’t an error. If you need real distinction, use “branded” or “newtype” patterns, or a small class (primitive obsession).

const u: UserId = "u1";
const o: string = u;           // fine: it's the same type

Aliases or interfaces?

In TypeScript, type and interface overlap for object shapes. interface can be extended and merged, type can also name unions and other forms. Follow your team’s convention.

Use aliases to name things, but don’t hide a complicated type behind a vague name. See also generics and union types.