Optional and readonly properties

📘What it really is

• Optional properties may be absent, marked with ?:

interface Customer {
  id: string;
  name: string;
  email?: string;
}

customer.email has the type string | undefined, so TypeScript makes you handle the missing case.

• Readonly properties can't be changed after the object is created:

interface Order {
  readonly id: string;
  readonly createdAt: string;
  status: Status;
}

Trying order.id = "X" is an error. Use it for things that must never change, like ids and creation times.

• readonly string[] (or ReadonlyArray<string>) prevents push, pop and sort on an array.

• Be honest about optional. Marking things optional "just in case" spreads checks everywhere. If every customer must have a phone number, make it required.

• Readonly is a check while writing code; it doesn't freeze the object at runtime.