ES Modules（namedとdefault）、再エクスポート

// file: math.ts
export function add(a: number, b: number): number { return a + b }
export const PI = 3.14159

// file: app.ts
import { add, PI } from "./math"
console.log(add(2, 3), PI) // 5 3.14159

// file: greet.ts
export default function greet(name: string) { console.log(`Hello, ${name}`) }

// file: app.ts
import greet from "./greet"
greet("Ada")

// file: color.ts
export default "black"
export const palette = [ "red", "green", "blue" ]

// file: app.ts
import baseColor, { palette } from "./color"
console.log(baseColor, palette.length)

// file: math/add.ts
export const add = (a: number, b: number) => a + b

// file: math/index.ts (barrel)
export { add } from "./add"
export * from "./more"

// file: app.ts
import { add } from "./math"
console.log(add(1, 2))