Not awaiting nested promise

It should be noted that I am pretty new to all of this asynchronous stuff.

I am trying to wait until the existence of a file is verified, whereby the script creates the file if need be, before updating the file. However, I cannot seem to figure out how to do it.

I know that I can use fs.writeFileSync, but I would prefer to make it asynchronous, so to guarantee that it does not block any user activity.

// this is now detectDriveInfo(), the entire function unedited, verbatim
async function detectDriveInfo(){
  const exec = require('child_process').exec
  let
    totalFreespace = 0,
    totalSize = 0,
    drives = []
  exec('wmic logicaldisk get freespace,name,size,volumename', (error, stdout)=>{
    stdout
      .trim()
      .split('\r\r\n')
      .map(value => value.trim().split(/\s{2,}/))
      .slice(1)
      .sort((a,b) => Number(a[0]) - Number(b[0]))
      .forEach(async (value, i, a) => {
        renderDriveInfo(...value)
        totalFreespace += Number(value[0])
        totalSize += Number(value[2])
        drives.push([value[1], Number(value[2]) - Number(value[0])])
        if (i === a.length-1) {
          renderDriveInfo(totalFreespace,'ALL',totalSize,'')
          updateConfigDrives(drives)
          await guaranteeData(drives) // this and its nested promises have to happen/complete
          updateData(drives)          // before this
        }
      })
  })
}

async function guaranteeData(drives){
  const fs = require('fs')
  if (!fs.existsSync('./data.json')) {
    let json = {}
    drives = drives.map(([v]) => v)
    drives.forEach(v => {
      json[v] = []
    })
    json = JSON.stringify(json, null, 2)
    await fs.writeFile('./data.json', json, 'utf8', (error)=>{
      if (error) throw error
      console.log('The file, data.json, has been created.')
      console.log(json)
    })
    return
  }
}

Console Logs

1. should come last
2. The file, data.json, has been created.
3. {
  "C:": [],
  "G:": [],
  "K:": [],
  "D:": [],
  "E:": [],
  "H:": [],
  "J:": [],
  "I:": [],
  "F:": []
}

What am I doing wrong?

You’re mixing callbacks and promises. The callback version of fs.writeFile returns undefined, which unfortunately is a value JavaScript will be perfectly happy to await for you (by not waiting for anything at all).

Use fs.promises.writeFile from the fs Promises API with no callback:

await fs.promises.writeFile('data.json', json, 'utf8')
console.log('The file, data.json, has been created.')
console.log(json)

You’ll probably also want to specify that the file should never be overwritten, in case it’s created between the existence check and the write:

await fs.promises.writeFile('data.json', json, {
  flag: 'wx',
  encoding: 'utf8',
})

Then use the non-synchronous equivalent of fs.existsSync:

if (await fs.promises.access('data.json', fs.constants.F_OK)
            .catch(err => err.code === 'ENOENT' || Promise.reject(err))) {

Or just skip the existence check entirely and rely on wx if creating the JSON isn’t too expensive.

forEach with an async action is always wrong, too, because forEach discards whatever its action returns, and the return value of an async function is critical. You need a regular loop to run serially, although it’s unclear where drives comes from:

for (const value of stdout) {
  // ??
}

await guaranteeData(drives)
console.log('should come last')
// updateData(drives)

Finally, a promise version of exec is required for detectDriveInfo() to resolve correctly. Right now the promise it returns doesn’t wait for the operation to complete either.

After seeing your code , i would suggest using a modern for … of loop instead, in which await will work as expected :

for (const value of stdout) {
  // Inside this await will work as you aspecting
}