nodejs writing to JSON into a specific path

Try this

// var serverId = client.guilds.cache.map(guild => guild.id); // getting server id 
var serverId = "server01"; // getting server id 
var lang = "en"; // lang
// var config = readConfigFile(); //reading file from config yes there is JSON.parse
const config = {servers: []} // assuming JS is JSON.parse of JSON
var server = {[serverId] : lang};

config.servers.push(server)
console.log(config)

Okay, I've got how to write to a path but, when I'm writing to this:

"servers":{

},

It writes something like this "servers":"[object Object][object Object]",

brackets go poof and instead of server id it writes object Object. Why?

var serverId = client.guilds.cache.map(guild => guild.id); // getting server id 
var lang = "en"; // lang
var config = readConfigFile(); //reading file from config yes there is JSON.parse
var server = {serverId : lang};
console.log(server);
config.servers = config.servers + server;
config = JSON.stringify(config);
saveConfigFile(config);

and the console.log(server); for some reason spits out {serverId: "en"}. Why serverId is not a variable?

It sounds like you're wanting to be able to load the content of your JSON file into your bot, make edits to it, and then write them back to file again. If so, something like the following would work for managing the file (without any error handling):

const fs = require('fs');

const pathToJsonFile = 'some/path/your-file.json';

function readConfigFile(){
  const asJavascript = JSON.parse(fs.readFileSync(pathToJsonFile,'utf8'));
  return asJavascript;
}

function saveConfigFile(configAsJavascript){
  fs.writeFileSync(pathToJsonFile,configAsJavascript);
}

function addServerToConfig(server){
  const config = readConfigFile();
  config.servers.push(server);
  saveConfigFile(config);
}

You could then call addServerToConfig() from your bot.

If you want to keep track of both servers and their language, I'd recommend either storing them as an array of objects, e.g.

"servers":[{"name":"server-name-or-id-or-whatever", "language": "pl"}]

Or just storing a single object instead of an array, since that way you can do lookups without having to search, e.g.

"servers": {"server-id-1": "pl", "server-id-2": "en"}

insatll the package npm install fs

Then you must call upon the FS manager

fs = require('fs);

"servers":[
            fs.readFileSync('filepathhere' , 'utf8') function(err, data){
             if (err) {throw err}
             var data = data;
              return data;
              };
             //for writing to those files 
            try {
        if(fs.existsSync(arrayname+".txt")) {
            console.log("The file exists.");
        } else {
            fs.appendFile(arrayname+".txt", arrayname.tostring(), function (err){
                if (err) throw err;
                    console.log('Saved!');
            });
            console.log('The file does not exist.');
     }
} catch (err) {
        console.error(err);
}

]

Hope this helps if there's any specific questions regarding the specific array please let me know

If you want add a new item to an Array use .push() function