const fs = require('fs');

const read_file = (fname, cb) => {
  fs.stat(fname, (err, stats) => {
    if(err) {
      cb(err, null);
    } else {
      fs.open(fname, 'r', (err, fd) => {
        if(err) {
          cb(err, null);
        } else {
          let inbuf = Buffer.alloc(stats.size);
          fs.read(fd, inbuf, 0, stats.size, null, (err, bytesRead, buffer) => {
            if(err) {
              cb(err, null);
            } else {
              cb(err, buffer);
            }
          });
        }
      });
    }
  });
}

read_file('test.txt', (err, result) => {
  if(err) {
    console.log(err);
  } else {
    console.log(`Result is ${result.toString()}`);
  }
});