TypeScript Interfaces vs Types: Complete Comparison Guide

Typescript Study Chapter · Official ReferenceAfzaal Suleman Knowledge Base

TypeScript provides two main ways to define custom types: interfaces and type aliases. While they often achieve similar results, they have distinct use cases and capabilities.

Key Differences

Interfaces: Can be extended, can implement multiple interfaces, better for object shapes
Types: Can create unions, intersections, primitives, more flexible for complex types

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Code Demonstration & Syntax

typescript

// Interface - extensible
interface User {
  name: string;
  age: number;
}

interface AdminUser extends User {
  permissions: string[];
}

// Type - flexible for unions
type ID = string | number;
type Status = "active" | "inactive" | "pending";

// Type - intersection
type Employee = User & {
  employeeId: string;
};

Output

// No output - type definitions only

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Chapter Rules & Key Notes

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Remember This

Use interfaces for object shapes that might be extended
Use type aliases for unions, intersections, and primitive aliases
Both can be used in most cases, but interfaces are preferred for public APIs

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Important Rules

Prefer interfaces when defining object shapes that need extension
Use types for unions, intersections, and complex type compositions
Interfaces support declaration merging, types do not

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Common Mistakes

Using types when interfaces would be more appropriate for object shapes
Not leveraging declaration merging with interfaces
Overcomplicating with type intersections when simple extension would work

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