javascript - 来自较早 promise 的控制台日志出现在来自最新 promise 的控制台日志之后

Question

这个问题在这里已经有了答案:





What is the explicit promise construction antipattern and how do I avoid it?

(3 个回答)


2年前关闭。




下面不是我的确切代码，但我只是想概述一下结构。当我运行代码时，我得到控制台日志的顺序是这样的:

'完成'

json

我希望这会反过来，因为为了让 function2 完成(并发送“完成”解析值)， function3 必须首先完成。
我想了解为什么它不能那样工作。

function1().then(() => {
      return function2()
    ).then((message) => {
      console.log(message)
    })

    function function2() {
      return new Promise((resolve, reject) => {
        fetch(url, {
            method: 'get',
            body: null,
            headers: {
              "Content-Type": "application/json; charset=UTF-8",
              "Accept": "application/json; charset=UTF-8",

            },
          })
          .then(res => res.json())
          .then((json) => {
            return function3(json)
          })

        resolve('Done')
      })
    }

    function function3(json) {
      console.log(json)
    }

Answer 1

您调用resolve在您的 fetch 之前甚至完成。
如果你把它移到另一个 then 就行了:

// ...
.then(res => res.json())
.then((json) => {
    return function3(json)
})
.then(() => {
    resolve('Done')
})

但其实整个 new Promise甚至没有必要因为 fetch已经返回了一个 promise !

// ...

function function2() {
    return fetch(url, { // It's important to actually *return* the promise here!
        method: 'get',
        body: null,
        headers: {
            "Content-Type": "application/json; charset=UTF-8",
            "Accept": "application/json; charset=UTF-8"
        }
    })
    .then(res => res.json())
    .then((json) => {
        return function3(json)
    })
    .then(() => {
        return 'Done'
    })
}

这可以使用 async 进一步简化。/ await :

// I moved this into a function because the global module-level code
// is not in an async context and hence can't use `await`.
async function main () {
  await function1()
  console.log(await function2())
}

async function function1 () {
  // I don't know what this does, you didn't include it in your code snippet
}

async function function2 () {
  const response = await fetch(url, {
    method: 'get',
    body: null,
    headers: {
      "Accept": "application/json; charset=UTF-8"
    }
  })
  
  const json = await response.json()
  await function3(json)

  return 'Done'
}

// At the moment this wouldn't have to be async because you don't do anything
// asynchronous inside, but you had `return function3(json)` in the original code,
// so I assume it is *going* to be async later.
async function function3 (json) {
  console.log(json)
}

// Don't forget to .catch any rejections here! Unhandled rejections are a pain!
main().catch(console.error)

(当我这样做时，我删除了 Content-Type header ，因为它对 GET 请求毫无意义。)