TypeScript · Narrowing and unions

Discriminated union

A shared literal field lets TypeScript narrow the whole object in a switch.

Syntax

type Shape = { kind: "circle"; r: number } | …

Example

type Shape =
  | { kind: "circle"; r: number }
  | { kind: "square"; size: number };
function area(shape: Shape) {
  return shape.kind === "circle" ? Math.PI * shape.r ** 2 : shape.size ** 2;
}

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Discriminated union

Example

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