A type is a promise; a boundary needs evidence

First, build a mental model

The compiler reasons about the evidence in your source. It cannot inspect tomorrow's network response. Treat external data as unknown, then narrow it with runtime checks.

1. JSON.parse returns an unchecked value; deliberately assign it to unknown.
2. Check object/null/array shape before reading fields.
3. Validate domain rules as well as primitive types. A negative quantity is still a number.
4. An assertion changes the compiler's belief without running a validator.

type Order = { id: string; quantity: number };
function decodeOrder(value: unknown): Order | null {
  if (typeof value !== 'object' || value === null || Array.isArray(value)) return null;
  if (!('id' in value) || typeof value.id !== 'string') return null;
  if (!('quantity' in value) || typeof value.quantity !== 'number') return null;
  if (!Number.isInteger(value.quantity) || value.quantity < 1) return null;
  return { id: value.id, quantity: value.quantity };
}
const payload: unknown = { id: 'demo-1', quantity: -2 };
const order = decodeOrder(payload);

1. Input

   payload is unknown: access is restricted until there is evidence.

2. Narrow

   Each guard removes invalid possibilities from the branch that continues.

3. Boundary

   The invalid quantity returns null. It never becomes a valid Order.