ts :: exhaustive type checking - `Extract`

TS-Exhaustive-Type-Check.ts

// Inspired from article
// https://medium.com/@wittydeveloper/typescript-super-types-62ca18810730

type Square = { kind: 'square'; side: number };
type Rectangle = { kind: 'rectangle'; width: number; length: number };
type Circle = { kind: 'circle'; radius: number };

type Shape = Square | Rectangle | Circle;
type ShapeKind = Shape['kind'];

// type AreaList = { [K in Shape['kind']]: (shape: Extract<Shape, { kind: K }>) => number };
type AreaList = { [K in ShapeKind]: (shape: Extract<Shape, { kind: K }>) => number };

const areaCalc: AreaList = {
  square: (s: Square) => s.side * s.side,
  rectangle: (s: Rectangle) => s.width * s.length,
  circle: (s: Circle) => (s.radius * s.radius * 22) / 7
};

// with destructuring
const areaCalc2: AreaList = {
  square: ({ side }) => side * side,
  rectangle: ({ width, length }) => width * length,
  circle: ({ radius }) => (radius * radius * 22) / 7
};

const computedAreas = [
  areaCalc['square']({ kind: 'square', side: 10 }),
  areaCalc['rectangle']({ kind: 'rectangle', width: 10, length: 10 }),
  areaCalc['circle']({ kind: 'circle', radius: 10 }),

  areaCalc2['square']({ kind: 'square', side: 10 }),
  areaCalc2['rectangle']({ kind: 'rectangle', width: 10, length: 10 }),
  areaCalc2['circle']({ kind: 'circle', radius: 10 })
];

console.log(computedAreas);