Namespaces in TypeScript

What are Namespaces?

Namespaces (previously called “internal modules”) organize related code into a single scope, preventing naming collisions. They’re primarily used for internal module organization in non-ES-module codebases.

Basic Namespace

namespace Validation {
  export interface StringValidator {
    isValid(s: string): boolean;
  }

  export class EmailValidator implements StringValidator {
    isValid(s: string): boolean {
      return s.includes("@");
    }
  }
}

// Usage
const emailValidator = new Validation.EmailValidator();

Nested Namespaces

namespace App {
  export namespace Models {
    export interface User { id: number; name: string }
  }
  export namespace Utils {
    export function formatDate(date: Date): string { return date.toISOString(); }
  }
}

When to Use Namespaces vs ES Modules

Feature	Namespaces	ES Modules
Scope	Single file/global	File-based
Tree-shaking	Limited	Excellent
Modern codebases	Legacy	Preferred
Bundler support	Limited	Full

Interview Questions

Easy: What is a namespace in TypeScript?

Medium: How do you export members from a namespace?

Hard: When should you use namespaces instead of ES modules?