In Node.js, what's "on"?

Question

In official doc, there is some sample code:

var req = http.request(options, function(res) {
  console.log('STATUS: ' + res.statusCode);
  console.log('HEADERS: ' + JSON.stringify(res.headers));
  res.setEncoding('utf8');
  res.on('data', function (chunk) {
    console.log('BODY: ' + chunk);
  });
});

I can understand it except one part: what's on in res.on? What's the difference between it and addListener?

Answer 1

As far as I'm aware, it's no different than addListener. There is some documentation on the event here: http://nodejs.org/docs/latest/api/events.html#emitter.on Both on and addListener are documented under the same heading. They have the same effect;

server.on('connection', function(stream) {
    console.log('someone connected!');
});

server.addListener('connection', function(stream) {
    console.log('someone connected!');
});

Answer 2

Both on and addListener are aliases for the same function.