异步函数返回Promise {< pending>}? [英] async function returns Promise { <pending> }?

问题描述

我具有以下异步功能:

async function readFile () {
  let content = await new Promise((resolve, reject) => {
    fs.readFile('./file.txt', function (err, content) {
      if (err) {
        return reject(err)
      }
      resolve(content)
    })
  })

  console.log(content)
}

readFile()

这很好.它将文件缓冲区按预期输出到控制台.但是现在，如果我尝试返回值:

This runs just fine. It outputs the file buffer to the console as expected. But now, if I try to instead return the value:

async function readFile () {
  let content = await new Promise((resolve, reject) => {
    fs.readFile('./file.txt', function (err, content) {
      if (err) {
        return reject(err)
      }
      resolve(content)
    })
  })

  return content
}

console.log(readFile())

我现在得到:

Promise { <pending> }

这是为什么?为什么可以在该函数内使用一个值，但是当您从该函数中将其返回时，它现在却是一个Promise?

Why is this? Why can you use a value inside that function but when you return it out of the function it's now a Promise?

您如何在正常的工作流程中实际使用此功能?例如，假设我要检查文件是否存在，然后读取文件，然后使用内容更新一些数据库，同步伪代码将如下所示:

How do you actually make use of this in a normal workflow? For example, lets say I wanted to check if a file exists, then read in the file, then update some database with the content, the synchronous pseudo code would look something like this:

if (fileExists(path)) {
  buffer = readFile(path)
  updateDatabase(buffer)
}

该工作流程由3个独立的异步操作组成.您如何使用 async/await 做这样的事情?是将整个脚本包装在 async 函数中的键吗?

That workflow consists of 3 individual async operations. How would you do something like this with async/await? Is the key that you have to have your entire script wrapped in an async function?

async function doSomething () {
  if (fileExists(path)) {
    buffer = readFile(path)
    updateDatabase(buffer)
  }
}

(请记住，这只是伪代码，但希望能引起我的注意).

(Keep in mind that is just pseudo-code but hopefully its gets my point across).

推荐答案

所有 async 函数均返回承诺，如注释中所述.因此，您可以像这样重新编写 readFile 函数:

All async functions return a promise as was mentioned in the comments. You could therefore re-write your readFile function like this:

function readFile() {
  return new Promise((resolve, reject) => {
    fs.readFile('./file.txt', function (err, content) {
      if (err) {
        return reject(err)
      }
      resolve(content)
    })
  })
}

然后您将通过 await 使用 readFile 的返回值:

You would then consume the return value of readFile via await:

console.log(await readFile()) // will log your actual file contents.

这种范例的通常工作流程是将异步操作分解为单独的函数，每个函数都返回一个Promise，然后在更广泛的 async 函数中运行它们，就像您建议的那样，但是使用 await s和一些错误处理，例如:

The usual workflow with this paradigm is to break your async operations into separate functions that each return a promise, and then run them all inside a broader async function, much like you suggest, but with awaits and some error handling like so:

async function doSomething () {
  try {  
    const fileCheck = await fileExists(path)

    if (fileCheck) {
      const buffer = await readFile(path)
      await updateDatabase(buffer)
      // Whatever else you want to do
    }
  } catch (err) {
    // handle any rejected Promises here.
  }
}

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