Google Cloud Function (HTTP trigger) to read the COM (comment) fields of a JPEG image

read_JPEG_COM.js

// Get a reference to the Cloud Storage component
var storage = require('@google-cloud/storage');

function findCOMmarkers(buffer) {
  var comment = [];
  
  for (var i=0; i < buffer.length; i++) {
    if (buffer[i] == '0xFF') {
      if (buffer[i+1] == '0xFE') { // JPEG COM marker
        comment.push(readcomment(buffer, i+4));
      };
    };
  };
  return comment;
};

function readcomment(buffer, pos) {
  //TODO(): a static length would prob help performance
  var commentString = [];

  for (true; pos < buffer.length; pos++) {
    if (buffer[pos] != '0xFF') {
      commentString.push(buffer[pos]);
    } else {
      break;
    };
  };
  return commentString;
};

function bytesToHex(bytes) {

    for (var hex = [], i = 0; i < bytes.length; i++) {
        hex.push((bytes[i] >>> 4).toString(16));
        hex.push((bytes[i] & '0xF').toString(16));
    }
    return hex.join("");
};


exports.readJPGCOM = function readbytes(req, res) {
  var pos = 0;
  var comment = 0;
  var buflength = 300; // all comments should be in the first 300 bytes ;-)
  
  var gcs = storage({
    projectId: process.env.GCP_PROJECT,
    keyFilename: "./camfunctions01-925a41b2138f.json"
  });

  // Reference an existing bucket object
  file = gcs.bucket('camimages').file('09/09_00_09.trig-01.jpg')
  
  file.createReadStream({
      start: 0,
      end: buflength
  })
    .on('error', function(err) {})
    .on('end', function(data) {
      while ( pos + 2 <= 300 ) {
        pos += 2;
      }
      res.send(); // required or Cloud Function will hang
    })
    .on('data', function(data) {
       comments = findCOMmarkers(data);
     });

  for (i=0; i < comments.length; i++) {
    console.log(String.fromCharCode.apply(null, comments[i]))
  };
  return;
};

// check if we are running in local Node.js or Cloud Functions
if (process.env['NODE_ENV'] != 'production') {
  var fs = require('fs');
  var path = "21_49_29.trig+00.jpg";
  var buflength = 200;
  var buffer = new Buffer(buflength);

  fs.open(path, 'r', function(status, fd) {
    if (status) {
      console.log(status.message);
      return;
    }
  
    fs.read(fd, buffer, 0, buffer.length, 0, function(err, num) {
    var comments = findCOMmarkers(buffer);
    for (j=0; j < comments.length; j++) {
        console.log(String.fromCharCode.apply(null, comments[j]));
    });
  });
};