Unions and intersections

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Unions and intersections#

A | B is “either”; A & B is “both”.

A union of two primitives.

type ID = string | number;
const id: ID = "abc";

Intersection combines object structures.

type Timestamped = {createdAt: Date};
type User        = {id: string; name: string};
type Row         = User & Timestamped;

A discriminated union pairs each member with a literal tag the operator switches on; the compiler narrows automatically.

type Shape =
  | {kind: "circle"; r: number}
  | {kind: "rect";   w: number; h: number}
  | {kind: "tri";    a: number; b: number; c: number};

The discriminator drives the narrow.

function area(s: Shape): number {
  switch (s.kind) {
    case "circle": return Math.PI * s.r ** 2;
    case "rect":   return s.w * s.h;
    case "tri":    return /* … */ 0;
  }
}

References#

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