javascript callback function with request and response

Question

The server needs to respond to a http get with all the data from separate file. Here I am using a csv parser.

   function sendFileData(req, res) {

      var result = []
      // Convert an csv file to Json entries
      for (var i = 0; i < 4; i++) {
        var dataArray = []
        csvFilePath = ...
        time = '"'
        value = '""'
        fs.createReadStream(csvFilePath)
         .pipe(csv({headers: ['date', 'time', 'value']}))
         .on('data', function (data) {
           date = ...
           time = ...
           value = ...
           dataArray.push('{' + date + time + value + '}')
         })
         .on('end', function () {
           var sensorData = '[' + dataArray + ']' 
           result.push(sensorData)
         })
      }
      res.send(result)
    }

Since the for loop takes some time to finish, the result is always [], so I consider to add setTimeout() and a callback function, but I feel the setTimeout is a bad approach.

   function sendFileData(req, res, callback) {

      var result = []
      // Convert an csv file to Json entries
      for (var i = 0; i < 4; i++) {
        var dataArray = []
        csvFilePath = ...
        time = '"'
        value = '""'
        fs.createReadStream(csvFilePath)
         .pipe(csv({headers: ['date', 'time', 'value']}))
         .on('data', function (data) {
           date = ...
           time = ...
           value = ...
           dataArray.push('{' + date + time + value + '}')
         })
         .on('end', function () {
           var sensorData = '[' + dataArray + ']' 
           result.push(sensorData)
         })
      }

    setTimeout(function () {
     callback(res, result)
    }, 1000)

}

function sendData (res, result) {
  res.send(result)
}

// calling function
sendFileData(req, res, sendData)

Is there a better way to send all the data after the csv-parser finished reading ?

Yes, don't send the result before the async stuff finishes, e.g., int he `end` event. — Dave Newton, Jun 22 '17 at 21:35
Call `res.send` **inside** the final callback (the `end` callback). See the linked question's answers for why. — T.J. Crowder, Jun 22 '17 at 21:36
Settimeout is definitely a bad approach. Have a look at the `async` library, should make your life easier — Chris Satchell, Jun 22 '17 at 21:38
@Deidara: No, you'll need to wait until you have all the results. Which is covered in at least one of the answers to the linked question. — T.J. Crowder, Jun 22 '17 at 21:47

user94559 · Accepted Answer · 2017-06-22T21:50:10.810

0

Send the response only when you have all the data. See the code below, which waits until the length of the result array is 4 (the number of iterations of the loop):

function sendFileData(req, res) {
  var result = []
  // Convert an csv file to Json entries
  for (var i = 0; i < 4; i++) {
    var dataArray = []
    csvFilePath = ...
    time = '"'
    value = '""'
    fs.createReadStream(csvFilePath)
     .pipe(csv({headers: ['date', 'time', 'value']}))
     .on('data', function (data) {
       date = ...
       time = ...
       value = ...
       dataArray.push('{' + date + time + value + '}')
     })
     .on('end', function () {
       var sensorData = '[' + dataArray + ']'
       result.push(sensorData)
       remaining -= 1
       if (result.length === 4) {  // all done
        res.send(result)
       } 
     })
  }
}

edited Jun 22 '17 at 21:50

answered Jun 22 '17 at 21:36

user94559

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can I put res.send() in a for loop? – Deidara Jun 22 '17 at 21:44
Oh, sorry. You'll need to only send the result if all iterations are done. – user94559 Jun 22 '17 at 21:45
See the updated code. – user94559 Jun 22 '17 at 21:50

javascript callback function with request and response

1 Answers1