Typescript Foundations / Objects & Interfaces

Type aliases

👉 A type alias lets you define a custom name for a type (including object types, unions, functions, etc.).

👉 It makes your code cleaner, more readable, and DRY (Don't Repeat Yourself).

✅ Basic Syntax

type AliasName = Type;

✍ Example 1: Primitive Type Alias

type Username = string;

let user1: Username = "Alice";
let user2: Username = "Tisha";

✍ Example 2: Union Type Alias

type Status = "pending" | "approved" | "rejected";

let orderStatus: Status = "approved";
// orderStatus = "done"; // ❌ Error

✍ Example 3: Array or Tuple Alias

type StringArray = string[];
type Point = [number, number];

const names: StringArray = ["Alice", "Bob"];
const coord: Point = [10, 20];

✍ Example 4: Function Type Alias

type Logger = (message: string, level?: "info" | "error") => void;

const log: Logger = (msg, level = "info") => {
  console.log(`[${level.toUpperCase()}]: ${msg}`);
};

✍ Example 5: Object Type Alias

type User = {
  name: string;
  age: number;
  isAdmin?: boolean;
};

const user: User = {
  name: "Codeveda",
  age: 21
};

🔄 Combined Example

✅ Summary

Feature

Example

Basic Alias

type ID = number

Object Alias

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

Union Types

`type Status = "open"

Function Alias

type Fn = (x: string) => void

Reusable Types

type Point = [number, number]

Combined Types

type AdminUser = Admin & Person

💡 Type Alias vs Interface

Feature

type

interface

Aliases for any type (primitives, unions, functions, etc.)

✅

❌

Extends/Implements for OOP

✅ via &

✅ with extends

Declaration merging

❌

✅

🔹 Use type for flexibility and unions

🔹 Use interface for class-like or extendable structures