simple_api_server.js

const url = require('url');
const http = require('http');

function getBody(req, res) {
  return new Promise(function inPromise(resolve, reject) {
    if (req.method !== "POST") {
      resolve();
    }

    res.body = '';
    req.on("data", (b) => {
      res.body += b;
    });

    req.on("end", () => {
      resolve(res.body);
    });

    req.on("error", reject);
  }).then((body) => {
    return body ? JSON.parse(body) : {};
  });
}

http.createServer((req, res) => {
  req.urlData = url.parse(req.url, true);

  Promise.resolve().then(() => {
    return getBody(req, res).then((body) => {
      const act = req.urlData.pathname;
      Object.assign(body, req.urlData.query);

      if (API.hasOwnProperty(act)) {
        return API[act](req, res, body);
      } else {
        return API.default(req, res, body);
      }
    });
  }).catch((e) => {
    console.error('catch', e);
    res.end(JSON.stringify({
      error: e.error_msg || "Неизвестная ошибка",
      code:  e.error_code || 500
    }));
  });
}).listen(8081);

const API = {
  '/echo': (req, res, body) => {
    res.writeHead(200, {
      'Content-Type': 'text/json; charset=utf-8',
      'Access-Control-Allow-Origin': '*',
    });

    res.end(body.input || 'empty input');
  },
  default: (req, res, body) => {
    console.log('pathname:', req.urlData.pathname);
    res.end('default');
  }
};