TypeScriptを1.xしか使ったことがない人のための基礎知識

function error(message: string): never {
  throw new Error(message);
}

function dice(): 1 | 2 | 3 {
  return (Math.floor(Math.random() * 3) + 1) as 1 | 2 | 3
}
const result = dice()
console.log(result)
// 1 or 2 or 3

interface Point {
  readonly x: number;
  readonly y: number;
}

var p: Point = { x: 1, y: 2 };
p.x = 3; // Error, p.x is read-only

enum Colors {
  Red = 'RED',
  Green = 'GREEN',
  Blue = 'BLUE'
}
console.log(Colors)
/*
{
  Red: "RED",
  Green: "GREEN",
  Blue: "BLUE"
}
*/

const million:number = 1_000_000
console.log(million)
// => 1000000

type Foo<T> = { -readonly [P in keyof T]-?: T[P] }

function foo(arr: readonly string[]) {
  arr.slice(); // okay
  arr.push("hello!"); // error!
}

function* count() {
  let i = 1
  while (true) {
    if (yield i++) {
      break
    }
  }
  return 4649
}

const iter = count()
let curr = iter.next()
while (!curr.done) {
  console.log(curr.value)
  curr = iter.next(curr.value === 3)
}
console.log(curr.value)
/* console結果
1
2
3
4639
*/

/**
 * - yields numbers
 * - returns strings
 * - can be passed in booleans
 */
function* count():Generator<number, string, boolean> {
  let i = 1
  while (true) {
    if (yield i++) {
      break
    }
  }
  return 4649 // stringではなくnumberなのでエラー
}

const iter = count()
let curr = iter.next()
while (!curr.done) {
  console.log(curr.value)
  curr = iter.next(curr.value === 3)
}
console.log(curr.value)

const foo:any = undefined
const bar = ():void => console.log('Hello!')
let a = foo !== null && foo !== undefined ? foo : bar()
let b = foo ?? bar()

// a と b は同じ結果
console.log(a) // => 'Hello!'
console.log(b) // => 'Hello!'

const foo:any = undefined
const r1 = foo?.trim()
console.log(r1)
// => undefined

const bar:any = undefined
const r2 = bar.trim()
console.log(r2)
// => エラーになる

interface User {
  isAdministrator(): boolean;
}

function foo(user: User) {
  if (user.isAdministrator) {
    // 処理
  }
}

// privateを使用する場合
class Person {
  private name: string

  constructor(name: string) {
    this.name = name
  }

  hello() {
    console.log(`Hello, ${this.name}!`)
  }
}

const ken = new Person('Ken')
ken.hello()
// => Hello, Ken!

console.log(ken['name'])
// => Ken

console.log(ken.name)
// Error!

// #を使用する場合
class Person {
  #name: string

  constructor(name: string) {
    this.#name = name
  }

  hello() {
    console.log(`Hello, ${this.#name}!`)
  }
}

const ken = new Person('Ken')
ken.hello()
// => Hello, Ken!

console.log(ken['#name'])
// undefined

console.log(ken.#name)
// Error!

export * as utilities from "./utilities.js";

// これを
a = a && b;
a = a || b;
a = a ?? b;

// このように書ける
a &&= b;
a ||= b;
a ??= b;

let a = 0
const b = 2

// a = a || b と同じ
a ||= b

console.log(a)
// 2