Exhaustiveness Checking in TypeScript

exhaustiveness-example.ts

// This is from Microsoft's presentation

type Shape =
  | {kind: 'circle'; radius: number}
  | {kind: 'rectangle'; w: number; h: number}
  | {kind: 'sqaure'; size: number}

// This function will throw ts-errors if obj ever comes in as something other than error
function assertNever(obj: never) {
  throw new Error('Unexpected object')
}

function getArea(shape: Shape) {
  switch (shape.kind) {
    case 'circle':
      return Math.PI * shape.radius ** 2
    case 'rectangle':
      return shape.w * shape.h
    case 'sqaure':
      return shape.size ** 2
  }
  // For example, if one of the cases above was missing, shape would be infered as that type
  // which would throw ts error
  assertNever(shape)
}

const shape: Shape = {kind: 'circle', radius: 10}
const area = getArea(shape)