Validation: External Data

Purpose: Ensure runtime type safety by validating all data from external sources. TypeScript types are compile-time only and erased at runtime.

When to use: Any time you receive data from outside your TypeScript code - API responses, JSON files, user input, database queries, environment variables, file uploads, or any external system.

Critical Understanding

TypeScript Types Are Compile-Time Only

This code has NO runtime safety:

interface User {
  id: string;
  email: string;
  age: number;
}