never

Let's imagine we want to get the area of circles and squares.

type Circle = { kind: "circle"; radius: number };
type Square = { kind: "square"; sideLength: number };
type Shape = Circle | Square;

function getArea(shape: Shape) {
  switch (shape.kind) {
    case "circle":
      return Math.PI * shape.radius ** 2;
    case "square":
      return shape.sideLength ** 2;
    default:
      const _exhaustiveCheck: never = shape;
      return _exhaustiveCheck;
  }
}

If someone would like to add triangles...

type Triangle = { kind: "triangle"; base: number; height: number };
type Shape = Circle | Square | Triangle;

The line const _exhaustiveCheck: never = shape; will show an error, saying that it should not happen. Very useful for exhaustive checking.

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• TypeScript: Documentation - Narrowing | The never type
• YouTube / Wes Bos: 🔥 TypeScript's never type explained