Exhaustive switch in TypeScript

The never trick that makes the compiler reject an unhandled case.

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

function area(shape: Shape): number {
  switch (shape.kind) {
    case "circle":
      return Math.PI * shape.r ** 2;
    case "square":
      return shape.s ** 2;
    default:
      const exhausted: never = shape;
      throw new Error(`unhandled: ${exhausted}`);
  }
}

Keywords and builtins used here

• Math
• Shape
• case
• const
• function
• never
• number
• return
• switch
• throw
• type

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