Type Aliases

Type aliases create custom names for any type, making code more readable and maintainable.

Basic Type Alias

Use the type keyword to create reusable type definitions:

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Union Types

Type aliases are perfect for union types (OR logic):

type Status = "pending" | "approved" | "rejected";
type ID = string | number;

let orderStatus: Status = "pending";
let userId: ID = "user123";
let productId: ID = 456;

When to Use Type Aliases

• Union types: string | number
• Primitive aliases: type ID = string
• Tuple types: type Point = [number, number]
• Function types: type Handler = (e: Event) => void
• Complex types: Combinations of the above

Type Alias vs Interface

Common Patterns

type Direction = "north" | "south" | "east" | "west";
type Theme = "light" | "dark" | "auto";

type Timestamped = { createdAt: Date };
type User = { userName: string };
type TimestampedUser = User & Timestamped;

type Nullable = T | null;
type Optional = T | undefined;

Common Mistakes

// ✗ Wrong - cannot redeclare
type ID = string;
type ID = number;  // Error!

// ✓ Correct - use union
type ID = string | number;

type Point = { x: number; y: number };

// ✗ Wrong - Point is a type, not a value
let p = new Point();

// ✓ Correct
let p: Point = { x: 10, y: 20 };

Summary

What's Next?

Next: Extending Interfaces to build complex types!