Why Validate?

TypeScript provides compile-time type safety, but can't validate external data at runtime. Data from APIs, user input, or databases arrives as any and TypeScript can't guarantee its shape.

External APIs are particularly problematic - you don't control them, and documentation is often outdated.

const userData = await response.json(); // 'any'
console.log(userData.profile.name.toUpperCase()); // Crashes if profile is null

Runtime validation creates a boundary between untrusted external data and your typed code:

const userData = UserSchema.parse(await response.json());
console.log(userData.profile.name.toUpperCase()); // Guaranteed to work

When validation fails, you can handle it gracefully instead of crashing.

Runtime validation might not be necessary if you control all the data or work on a small team with insight into all changes.