プロミスマスターメモ

プロミスの基本的な書き方

const prA = () => {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      console.log('prA')
      resolve()
    }, 1000)
  })
}
const prB = () => {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      console.log('prB')
      resolve()
    }, 1000)
  })
}
const prC = () => {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      console.log('prC')
      resolve()
    }, 1000)
  })
}

Promise.resolve()
  .then(prA)
  .then(prC)
  .then(prB)

for文を交えた書き方

const prA = () => {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      console.log('prA')
      resolve()
    }, 1000)
  })
}
const prB = () => {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      console.log('prB')
      resolve()
    }, 1000)
  })
}

const promises = []

const prX = (num) => {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      console.log(num)
      resolve()
    }, 1000)
  })
}

for (let i = 0; i < 3; i++) {
  promises.push(prX(i))
}

Promise.all(promises)
  .then(prA)
  .then(prB)