async function returns Promise { }?

Question

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 {  }

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).

user_13796662 · Answer

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);

async function returns Promise { <pending> }?

2 Answers