Optional and Readonly Properties

Example

type Address = {
  line1: string;
  line2?: string;
  city: string;
  landmark?: string;
};

function format(a: Address): string {
  const parts = [a.line1, a.line2, a.city];
  if (a.landmark) parts.push("near " + a.landmark);
  return parts.filter(Boolean).join(", ");
}

console.log(format({ line1: "12 MG Road", city: "Pune" }));
console.log(format({ line1: "Flat 5B", line2: "Lake View Apts", city: "Kochi", landmark: "Metro" }));

Output

12 MG Road, Pune
Flat 5B, Lake View Apts, Kochi, near Metro

Example

A readonly property can be set when the object is created but not changed later (TS2540).

type Order = {
  readonly id: string;
  status: string;
};

const order: Order = { id: "OD-1", status: "PLACED" };
order.status = "SHIPPED"; // fine
order.id = "OD-2";        // error

Compiler output (tsc)

Line 8: error TS2540: Cannot assign to 'id' because it is a read-only property.