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Discriminated union: shape area

The Shape type describes a circle and a rectangle; the kind field tells which one it is. Add a square { kind: "square"; size: number } to it and write area(shape) that returns the area of any shape.

It helps if TypeScript reminds you about area when another shape is added one day: add an exhaustiveness check with never.

type Shape =
  | { kind: "circle"; radius: number }
  | { kind: "rect"; width: number; height: number };

function area(shape: Shape): number {
  // ...
  return 0;
}

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Tests (6)

Type tests are checked by the TypeScript compiler in strict mode. Expect<Equal<A, B>> passes when the types match exactly, and // @ts-expect-error passes when the next line does not compile. The first run downloads the compiler, so it takes a few seconds.

types

type _ = Expect<Equal<Shape["kind"], "circle" | "rect" | "square">>;

○
types
```
area({ kind: "square", size: 3 });
```

types

// @ts-expect-error - a circle has a radius, not a width
area({ kind: "circle", width: 2 });

expect(Math.abs(area({ kind: 'circle', radius: 2 }) - Math.PI * 4) < 1e-9).toBe(true);

expect(area({ kind: 'rect', width: 3, height: 4 })).toBe(12);

expect(area({ kind: 'square', size: 5 })).toBe(25);