I am developing a module of MM.
By the way, I want to control one of these modules by an external client, and I want to do that control method by REST API method.
For example, if an external client sends a POST API to MM2, the corresponding message is displayed on the MM2 screen.
I don’t know because I haven’t read the MM2 code in detail, but as far as I’ve checked, the server also automatically turns on when running MM2, so I want to process REST API communication on that server as well, but which file should I modify and put a phrase related to REST API communication in order that?

server.js?
server_function.js?

Examples of API codes that I want to put in are these.
The API code is an API that stores the message as a txt file when a message is transmitted in a POST method.

app.post('/api/saveMessage', (req, res) => {
    const message = req.body.message;

    if (!message) {
        res.status(400).send('Message not provided.');
        return;
    }

    fs.unlink(`appid.txt`, (err) => {
        fs.appendFile(`appid.txt`, message + '\n', (err) => {
            if (err) {
                console.error('Error saving message:', err);
                res.status(500).send('Error saving message');
            } else {
                res.send('The message was saved successfully..');
            }
        });
    });
});

I want to open the server with the first IP address of the wlan0 network by putting ifaces.wlan0[0].address in the address part of the config.js file. I modified the code to do this, but I get an error on the screen.

const os = require("os");

const ifaces = os.networkInterfaces();
const wlan0Address = ifaces.wlan0[0].address;

let config = {
    address: wlan0Address, // The first IP address of the wlan0 network interface
    port: 8080, 
    // etc...
};

If I write the code as above, I don’t know why, but when I open the server, I don’t see any module screen on the web server. So I thought the modules were a problem, so I modified the code as follows, and the modules come out well from the web server at that address.

/*const os = require("os");
const ifaces = os.networkInterfaces();
const wlan0Address = ifaces.wlan0[0].address;
*/
let config = {
    address: "xxx.xxx.xxx.xxx", // static address
    port: 8080, 
    // etc...
};

The conclusion that I came up with through this process is

const os = require("os");
const ifaces = os.networkInterfaces();
const wlan0Address = ifaces.wlan0[0].address;

I think the code in the above part is problematic, I want to know exactly what the problem is with this code, and how to fix the code to solve it.

In addition, since English is not the main language, I hope you understand that my writing may have some grammatical errors.