Clean Boundaries

B1: Validate At The Edge

unknown

// Bad - assertion lets invalid data into the system
const user = JSON.parse(responseText) as User;

// Good - boundary handles parsing, validation, and source context
const user = parseUserResponse(responseText);

function parseUserResponse(responseText: string): User {
  const payload = parseJson(responseText, "user response");
  return parseUser(payload);
}

B2: Make Configuration Typed

B3: Do Not Leak Vendor Types

type PaymentReceipt = {
  id: string;
  totalCents: number;
};

interface Payments {
  charge(request: ChargeRequest): Promise<PaymentReceipt>;
}

B4: Boundary Tests

• Parser rejects invalid external shapes.
• Adapter maps SDK responses into domain types.
• Missing config fails with a useful message.
• Browser/storage fallbacks behave consistently.

Common Mistakes

• Letting

  any

  from a client library spread through application code.
• Repeating JSON shape checks in many call sites.
• Treating generated API models as domain models by default.
• Hiding unsafe parsing behind helper names like

  getUser()

  without validation.