Reading file from MongoDB GridFS on Server Side in NodeJS Application

Question

I want to read file from MongoDB GridFs and attach it to mail sent from node mailer. So I need to read this file at server side, what this means is I do not have access to http request or response object and I can not pipe the read stream to response suggested in multiple places over the internet.

My code to read the file and send as buffer is as below -

let _fetchFileById = async (fileId, options) => {
  let db = options.db
    , gfs = Grid(db, mongo)
    , filename = fileId
    , file_buf = new Buffer('buffer');
  return new Promise((resolve,reject) => {
    gfs.collection('jobdescription')

    let readstream = gfs.createReadStream({
      filename: filename
      , root: 'jobdescription'
    })

    readstream.on('data', function (chunk) {
      console.log("writing!!!");
      // file_buf.push(chunk)
      if (!file_buf)
      file_buf = chunk;
      else file_buf = Buffer.concat([file_buf, chunk]);
    });
    readstream.on('end', function () {
      // var buffer = Buffer.concat(file_buf);
      db.close();
      console.log(`returning`)
      // console.log(file_buf)
      resolve(file_buf)
    })
  })
}

when I am using the file_buf as input to the attachment in node mailer i am getting below errror -

TypeError [ERR_INVALID_ARG_TYPE]: The "chunk" argument must be one of type string or Buffer. Received type object TypeError [ERR_INVALID_ARG_TYPE]: The "chunk" argument must be one of type string or Buffer. Received type object . Any help or pointers would be really appreciated.

Answer 1

Finally i did not find a way to read file and return it directly from method, however I did a small workaround by reading it from db and storing it in local directory temporarily, once i attach it to nodemailer I intend to delete it. I am posting the method here, just in case it helps some one -

exports.downloadFile = async (req, fileName) => {
  let db = req.headers.options.db
    , gridfs = Grid(db, mongo)

  gridfs.collection('jobdescription')
  if (gridfs) {
    let fsstreamwrite = fs.createWriteStream(
      path.join(process.cwd(), `./app/jobdescriptions/${fileName}`)
    )
    let readstream = gridfs.createReadStream({
      filename: fileName
    })
    readstream.pipe(fsstreamwrite);
    return readstream.on("close", file => {
      console.log("File Read successfully from database");
      return file;
    })
  } else {
    console.log("Sorry No Grid FS Object");
  }
}