How to know if instance of `http.Server` in node is already listening

Simple

if (server.listening) { # }

https://nodejs.org/api/net.html#net_server_listening

Elaboration

• net.Server has a listening attribute indicating whether the server is listening
• http.Server inherits net.Server

When using an instance of http.createServer the address() function is available to the server instance. This indicates whether or not the actual server is listening and available.

Until listen() is called on the server instance address() returns a null because the instance itself does not yet have an address it is listening on.

Once listen() is called though, address() will return an object.

Example

var http = require('http');
var server = http.createServer();

console.log(server.address()); // This will be null because listen() hasn't been called yet

server.listen(3000, 'localhost', function() { 
    console.log('listening'); 
});

console.log(server.address()); // { address: '127.0.0.1', family: 'IPv4', port: 3000 }

Summary

If server.address() is null, your instance is not running.
If server.address() returns an object, your instance is running.