Ejemplo de un simple servidor web con Nodejs:

const http = require('http');

const hostname = '127.0.0.1';
const port = 3000;

const server = http.createServer((req, res) => {
  res.statusCode = 200;
  res.setHeader('Content-Type', 'text/plain');
  res.end('Hello World\n');
});

server.listen(port, hostname, () => {
  console.log(`Server running at http://${hostname}:${port}/`);
});

Diferencias entre callbacks, promesas, generadores, async/await

// callback

const authenticateUser = (ctx, next) => {

  User.findOne({
    name: ctx.req.user.dataValues.name
  }, (err, user) => {
    if (err) return next(err)

    if (user) {
      return user.veryfyPassword(ctx.request.body.password, (err, isPasswordMatch) => {
        if (err) return next(err)

        if (isPasswordMatch) {
          return issueToken(user._id, (err, token) => {
            if (err) return next(err)
            ctx.body = {
              status: 200,
              token: token,
            }
          })
        }
        ctx.body = {
          status: 401,
        }
      })
    }
    ctx.body = {
      status: 401,
    }
  })
}

// Promesa

const authenticateUser = (ctx, next) => {
  User.findOne({
      name: ctx.req.user.dataValues.name
    })
    .bind({})
    .then(user => {
      this.user = user
      if (user)
        return user.veryfyPassword(ctx.request.body.password)
      return Promise.resolve(false)
    })
    .then(isPasswordMatch => {
      if (isPasswordMatch)
        return issueToken(this.user._id)
      return Promise.resolve(null)
    })
    .then(token => {
      if (token)
        return ctx.body = {
          status: 200,
          token: token,
        }
      ctx.body = {
        status: 401,
      }
    })
    .catch(err => {
      next(err)
    })
}

//generadores

const authenticateUser = (ctx, next) => {

  Promise.coroutine(function* () {
    try {
      const user            = yield User.findOne({ name: ctx.req.user.dataValues.name })
      const isPasswordMatch = (user) ? yield user.veryfyPassword(ctx.request.body.password) : false 
      const token           = (isPasswordMatch) ? yield issueToken(user._id) : false

      if (token) {
        ctx.body = {
            status: 200,
            token:  token,
        }
      }

      ctx.body = {
        status: 401,
      }
    } catch (err) {
      next(err)
    }
  })()
}

// async/await

const authenticateUser = async (ctx, next) => {

  try {
    const user            = await User.findOne({ name: ctx.req.user.dataValues.name })
    const isPasswordMatch = (user) ? await user.veryfyPassword(ctx.request.body.password) : false 
    const token           = (isPasswordMatch) ? await issueToken(user._id) : false

    if (token) {
        ctx.body = {
            status: 200,
            token:  token,
        }
        return
    }

    ctx.body = {
      status: 401,
    }
  } catch (err) {
    next(err)
  }
}