Typescript/型の定義

let price: number
let isValid: boolean
let username: string

function calcTax(country: string, price: number, unit: number): number{
	return price * unit * 0.1
}

let tax = calcTax("aa", 2000, 5)
	
interface Rectangle {
	kind: "四角形",
	width: number,
	height: number
}

interface Circle {
	kind: "¥",
	radius: number
}

type Shape = Rectangle | Circle

function area(shape: Shape){
	switch(shape.kind){
		case "四角形": return shape.height * shape.width
		case "円": return shape radius * shape.radius * Math.PI
	}
}

const myRectangle: Rectangle = {
	kind: "四角形",
	width: 100,
	height: 30
}

const myCircle: Circle = {
	kind: "円",
	radius: 50
}

console.log(area(myCircle))