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awaitできるsetTimeoutを1行で書く方法

Last updated at Posted at 2019-10-28

awaitすることができるsetTimeoutを1行で書くイディオムです。

await new Promise(resolve => setTimeout(resolve, 3000)) // 3秒待つ

// ... 3秒後の処理 ...

おまけ: よくある長めの実装からの変形手順

// よくある長めの実装
const sleep = () => new Promise(resolve => {
  setTimeout(() => {
    resolve()
  }, 3000)
})
await sleep()

// ステップ1: () => { resolve() } は resolve に短縮する
const sleep = () => new Promise(resolve => {
  setTimeout(resolve, 3000)
})
await sleep()

// ステップ2: resolve => {} は resolve => に短縮する
const sleep = () => new Promise(resolve => setTimeout(resolve, 3000))
const promise = sleep()
await promise

// ステップ3: sleep()は関数でなくていいので値にする
const promise = new Promise(resolve => setTimeout(resolve, 3000))
await promise

// ステップ4: 完成
await new Promise(resolve => setTimeout(resolve, 3000))
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