Cannot get all output of child_process in nodejs

var spawn = require('child_process').spawn;
var child = spawn('/path/to/my/script');                                                                  

var stdout = '';   
var stderr = '';                                                                                                                                                                 
child.stdout.on('data', function (data) {                                                           
  stdout += data;
});

child.stderr.on('data', function (data) {                                                           
  stderr += data;
});

child.on('exit', function (code) {                                                                  
  console.log('child process exited with code ' + code); 
  var runner = new Runner({stdout: stdout, stderr: stderr});
  runner.save().then(function() {
     res.redirect('/');
  });                                
});   

I found that some time not all stdout/stderr will be saved.

Only part of stdout/stderr is saved.

What is wrong?

via Sato