mc-server-ping.js

// deps
const net = require("net");

const makeVarInt = (value) => {
    const buf = [];
    do {
        let cur = value & 0b01111111;
        value >>>= 7;
        if(value != 0) {
            cur |= 0b10000000;
        }
        buf.push(cur);
    } while(value != 0);
    return Buffer.from(buf);
};

const makeString = (string) => Buffer.concat([makeVarInt(string.length), Buffer.from(string, "utf-8")]);

// :-/
const makeUShort = (value) => {
    const buffer = Buffer.alloc(2);
    buffer.writeUInt16LE(value);
    return buffer;
};

const sendPacket = (socket, ...parts) => {
    socket.write(makeVarInt(parts.reduce((a, c) => a + c.length, 0)));
    parts.forEach(part => socket.write(part));
};

const skipVarInt = (buffer, offset) => {
    while((buffer[offset] & 0b10000000) != 0) {
        offset++;
    } 
    return offset + 1;
};

module.exports = (host, port, options) => new Promise((resolve, reject) => {

    const socket = new net.Socket();
    socket.setTimeout(options?.timeout ?? 5000);
    let ready = false, chunks =[];

    // net.Socket docs wrt timeout event:
    // > The user must manually close the connection.
    socket.on("timeout", () => {
        socket.destroy();
    });

    socket.on("error", (error) => {
        reject("Network error occurred: " + error);
    });

    // catch-all error handling :)
    socket.on("close", () => {
        if(!ready) {
            reject("The socket closed before enough data could be received");
        } else {
            const buffer = Buffer.concat(chunks);
            const dataStart = skipVarInt(buffer, skipVarInt(buffer, skipVarInt(buffer, 0))); // evil stupidity
            const data = buffer.slice(dataStart);
            try {
                resolve(JSON.parse(data));
            } catch(error) {
                reject("Could not parse response or none received");
            }
        }
    });

    socket.on("data", (data) => {
        chunks.push(data);
    });

    socket.connect({host, port});

    // build handshake packet
    sendPacket(
        socket, 
        makeVarInt(0), // packet ID
        makeVarInt(-1), // protocol version (-1 = pinging)
        makeString(host), // hostname + port even though vanilla server ignores these...
        makeUShort(port),
        makeVarInt(1), // target state (1 = status ping)
    );

    sendPacket(
        socket,
        makeVarInt(0) // packet ID
    );

    ready = true;        

    // *crickets*

});