Union & Intersection Types

Union Types (|)

A value can be one of several types.

type Status = "success" | "error" | "pending";
let s: Status = "success"; // OK
s = "loading"; // ❌ Not assignable

Intersection Types (&)

A value must satisfy all constituent types.

interface A { a: string; }
interface B { b: number; }
type AB = A & B; // must have both a and b
const obj: AB = { a: "foo", b: 123 };

Practical example – API response

type Success = { ok: true; data: string; };
type Failure = { ok: false; error: string; };
type ApiResult = Success | Failure;

function handle(r: ApiResult) {
  if (r.ok) {
    console.log(r.data); // `data` is guaranteed here
  } else {
    console.error(r.error);
  }
}