Type Aliases vs Interfaces

What is a Type Alias?

A type alias gives a name to any type:

type UserID = string;
type Status = "active" | "inactive";

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

You can use type for primitives, unions, intersections, and functions.

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What is an Interface?

An interface describes the structure of an object:

interface User {
  id: number;
  name: string;
}

Interfaces are extendable and composable, ideal for object shapes and class contracts.

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Key Differences

Feature	type	interface
Objects	✅	✅
Primitives, unions, etc.	✅ (strings, unions, functions)	❌ only objects
Extendable	✅ via intersections (&)	✅ via extends
Declaration merging	❌ no	✅ supported
Better for classes	❌ not ideal	✅ recommended

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When to Use type vs interface

Use interface when:

• Defining object structures
• Building APIs, classes, or data models
• You benefit from declaration merging

Use type when:

• Creating unions, tuples, or function signatures
• Combining multiple types with | or &
• Aliasing a primitive or non-object structure

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Can You Combine Them?

Yes — you can use both together:

interface Person {
  name: string;
}

type WithAge = Person & { age: number };

> You can even use a type inside an interface or vice versa.

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Summary

• type is more flexible — can alias anything
• interface is better for structured, extendable object types
• Use interface for long-lived objects and APIs
• Use type for complex unions, intersections, and primitives

Next up: Working with Enums — the right way to create named constants and semantic values.