Node.JS: debug to know one object's prototype name

I am reading the code of a chat project as a demo for socket.io

In index.js, here is a snippet:

var app = require('express')();
var http = require('http').Server(app);
var io = require('socket.io')(http);

So I use Node.JS debugger:

var app = require('express')();
var http = require('http').Server(app);
var io = require('socket.io')(http);

// add:
debugger;
watch("io");

The command is:

node debug index.js

It stops at var io = ... But I don't know how to get the information of the object "io". Actually when I type io in the terminal, it says: io is not defined.

if I type http, it will output the definition of http.

If I type app, it says: app is not defined.

I am confused. How to do it?