TypeScript: Interfaces and Type Aliases

1. Interface

An interface is the most common way to define the structure of an object.

interface User {
  readonly id: number; // Read-only
  name: string;
  age?: number; // Optional property
}

const user1: User = { id: 1, name: "John" };

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2. Type Alias

A type alias assigns a new name to a type, making it easier to reuse. It is particularly useful when dealing with complex union types.

type ID = string | number;
type Point = { x: number; y: number };

const myId: ID = 123;

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3. Interface vs Type

While both can be used in most situations, there are a few key differences:

Feature	Interface	Type
Extensibility	Inheritance via extends	Uses & (Intersection)
Declaration Merging	Automatically merged when redefined	Not possible (causes an error)
Primary Use Case	Defining the structure of objects	All types including unions and primitives

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4. Defining Function Types

You can define the parameter and return types of a function using an interface or a type alias.

interface MathFunc {
  (x: number, y: number): number;
}

const add: MathFunc = (a, b) => a + b;