mpv.js

const net = require("net");
const crypto = require("crypto");
const process = require("process");

function random() {
  return crypto.randomBytes(4).readUInt32LE(0);
}

function connect() {
  const socket = process.env.SOCKET || "/tmp/mpv.sock";
  const s = new net.Socket();
  const handlers = {};

  function command(x) {
    const requestId = random();
    s.write(JSON.stringify({ command: x, request_id: requestId }) + "\n");
    return new Promise(resolve => {
      handlers[requestId] = resolve;
    });
  }

  const c = new Promise(resolve => {
    s.on("data", data => {
      const json = JSON.parse(data.toString());
      if (json.request_id && handlers[json.request_id]) {
        handlers[json.request_id](json);
        delete handlers[json.request_id];
      }
    });

    s.connect(socket, () => {
      resolve({
        command
      });
    });
  });

  s.on("close", function() {
    console.log("Connection closed");
  });

  return c;
}

module.exports = { connect };

test-client.js

const connect = require("./mpv").connect;

async function main() {
  const mp = await connect();
  const volume = await mp.command(["get_property", "volume"]);
  console.log(volume); // { data: 100, request_id: 4139289790, error: 'success' }
}

main();