javascript - 等待一次获取完成后再开始下一次获取

Question

我有一个要发送到谷歌云的数据列表。我当前的代码如下所示:

const teams = ['LFC', 'MUFC', 'CFC'];

teams.forEach(team => {
    fetch({
      url: URL,
      method: 'PUT',
      body: team
    });
})

这适用于一个团队，但如果发送多个文件并且文件较大，则会超时。我发送的是图像而不是字符串。为了解决这个问题，我需要一个一个文件地POST数据，并等待上一个POST完成，然后再发送下一个文件。谁能建议最好的方法吗？

值得注意的是，我无法控制文件的数量已上传。

Answer 1

使用 reduce 代替 forEach，并使用 .then()。

以下代码会将最后一个 fetch 的 Promise 存储在 acc(reduce 的累加器参数)中，并附加新的 >fetch 在 then 监听器中，以确保之前的 fetch 已完成:

const teams = ['LFC', 'MUFC', 'CFC'];

teams.reduce((acc, team) => {
  return acc.then(() => {
    return fetch({
      url: URL,
      method: 'PUT',
      body: team
    });
  })
}, Promise.resolve())
  .then(() => console.log("Everything's finished"))
  .catch(err => console.error("Something failed:", err))

//Simulate fetch:
const fetch = team => new Promise(rs => setTimeout(() => {rs();console.log(team)}, 1000))

const teams = ['LFC', 'MUFC', 'CFC'];

teams.reduce((acc, team) => {
  return acc.then(() => {
    return fetch({
      url: URL,
      method: 'PUT',
      body: team
    });
  })
}, Promise.resolve())
  .then(() => console.log("Everything's finished"))
  .catch(err => console.error("Something failed:", err))

您甚至可以为其编写通用辅助函数:

const teams = ['LFC', 'MUFC', 'CFC'];

const promiseSeries = (arr, cb) => arr.reduce((acc, elem) => acc.then(() => cb(elem)), Promise.resolve())

promiseSeries(teams, (team) => {
  return fetch({
    url: URL,
    method: 'PUT',
    body: team
  })
})
  .then(() => console.log("Everything's finished"))
  .catch(err => console.error("Something failed:", err))

//Simulate fetch:
const fetch = team => new Promise(rs => setTimeout(() => {rs();console.log(team)}, 1000))

const teams = ['LFC', 'MUFC', 'CFC'];

const promiseSeries = (arr, cb) => arr.reduce((acc, elem) => acc.then(() => cb(elem)), Promise.resolve())

promiseSeries(teams, (team) => {
  return fetch({
    url: URL,
    method: 'PUT',
    body: team
  })
})
  .then(() => console.log("Everything's finished"))
  .catch(err => console.error("Something failed:", err))

或者，更好的是，如果可以的话(这是 ES2017)，使用 async/await (它更具可读性):

const teams = ['LFC', 'MUFC', 'CFC'];

async function upload(teams){
  for(const team of teams){
    await fetch({
      url: URL,
      method: 'PUT',
      body: team
    });
  }
}

upload(teams)
  .then(() => console.log("Everything's finished"))
  .catch(err => console.error("Something failed:", err))

//Simulate fetch:
const fetch = team => new Promise(rs => setTimeout(() => {rs();console.log(team)}, 1000))

const teams = ['LFC', 'MUFC', 'CFC'];

async function upload(teams) {
  for (const team of teams) {
    await fetch({
      url: URL,
      method: 'PUT',
      body: team
    });
  }
}

upload(teams)
  .then(() => console.log("Everything's finished"))
  .catch(err => console.error("Something failed:", err))