promise-memo.js

promise-memo.js

// 並列実行
;(async () => {
  const sleepTimes = [1000, 2000, 3000]

  // この時点で実行開始される
  const promises = sleepTimes.map((num) => new Promise((resolve, _reject) => {
    setTimeout(() => {
      const now = new Date()
      console.log(`${num} msec later. ${now}`)
      resolve(now)
    }, num)
  }))

  const values = await Promise.all(promises)
  console.log(`end. ${values}`)
})()


// 直列実行
;(async () => {
  const sleepTimes = [1000, 2000, 3000]

  // まだ実行してほしくないので、promiseオブジェクトではなくpromiseを返す関数を返す
  const promises = sleepTimes.map((num) => () => new Promise((resolve, _reject) => {
    setTimeout(() => {
      console.log(`${num} msec sleeping...`)
      const now = new Date()
      console.log(`${num} msec later. ${now}`)
      resolve(now)
    }, num)
  }))

  const values = await promises.reduce((acc, val) => acc.then(val), Promise.resolve()).catch(console.error)
  console.log(`end. ${values}`)
})()

// 直列実行 async版
;(async () => {
  const sleepTimes = [1000, 2000, 3000]
  const sleep = (msec) => {
    return new Promise(resolve => {
      setTimeout(resolve, msec)
    })	
  }

  // まだ実行してほしくないので、promiseオブジェクトではなくpromiseを返す関数を返す
  const promises = sleepTimes.map((num) => (async () => {
    console.log(`${num} msec sleeping...`)
    await sleep(num)
    const now = new Date()
    console.log(`${num} msec later. ${now}`)
    return now
  }))

  const values = await promises.reduce((acc, val) => acc.then(val), Promise.resolve()).catch(console.error)
  console.log(`end. ${values}`)
})()