CODEWITHSUNDEEP
javascriptnode.jsasynchronouspromise
Jake Wilson
Jake Wilson

Reputation: 91183

async function returns Promise { <pending> }?

I have the following async function:

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()

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())

I now get:

Promise { <pending> }

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?

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)
}

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)
  }
}

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

Upvotes: 9

Views: 13532

Answers (2)

Praveen Mishra
Praveen Mishra

Reputation: 11

const serchContentXmlFile = async (path, content) => {
  return new Promise((resolve, reject) => {
    fs.readFile(path, function (err, data) {
      if (err) {
        return reject(err)
      }
      resolve(data.indexOf(content))
    })
  })
}

await serchContentXmlFile("category.xml",xmlUrl);

Upvotes: 0

Mark McKelvy
Mark McKelvy

Reputation: 3536

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)
    })
  })
}

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

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

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.
  }
}

Upvotes: 8

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