day-1-type-narrowing-using-type-guards.ts

//100DaysOfCode - Day 1

type Rectangle = {
  width: number
  height: number
}

type Circle = {
  radius: number
}

const pi: number = 3.1415_9265_3589_7932_38 // yes you can use underscores to make a long number readable

function getArea(shape: Rectangle | Circle): number {
  //we are accepting Rectangle or Circle types
  //type narrowing using type guard
  if (isCircle(shape)) {
    //so this is shape is of Circle type
    return pi * Math.pow(shape.radius, 2)
  }
  return shape.width * shape.height
}

//type guard using type predicate
function isCircle(shape: Rectangle | Circle): shape is Circle {
  return (shape as Circle).radius !== undefined
}

//type guard using type predicate
function isRectangle(shape: Rectangle | Circle): shape is Rectangle {
  return (shape as Rectangle).width !== undefined && (shape as Rectangle).height !== undefined
}

const c: Circle = { radius: 20 }
const r: Rectangle = { width: 10, height: 8 }

console.log(getArea(c))
console.log(getArea(r))