listing-3.9-callbacks-named-functions.js

var http = require("http"),
  request = require('request'),
  fs = require('fs');

var serviceRootUrl = 'http://localhost:8686';

http.createServer(function (req, res) {
  console.log('New incoming client request...');

  if (req.url === '/log') {
    getTemperature(res);  //#A

  } else {
    res.writeHeader(200, {"Content-Type": "text/plain"});
    res.write('Please use /log');
    res.end();
  }

}).listen(8787);

function getTemperature(res) {  //#B
  request({url: serviceRootUrl + '/temperature', json: true}, function (err, resp, body) {
    if (err) throw err;
    if (resp.statusCode === 200) {
      console.log(body);
      var temp = body.temperature;

      getLight(res, temp);  //#C
    }
  });
}

function getLight(res, temp) {
  request({url: serviceRootUrl + '/light', json: true}, function (err, resp, body) {
    if (err) throw err;
    if (resp.statusCode === 200) {
      console.log(body);
      var light = body.light;

      logValuesReply(res, temp, light); //#D
    }
  });
}

function logValuesReply(res, temp, light) {
  var logEntry = 'Temperature: ' + temp + ' Light: ' + light;
  fs.appendFile('log.txt', logEntry + '\n', encoding = 'utf8', function (err) {
    if (err) throw err;
    res.writeHeader(200, {"Content-Type": "text/plain"}); //#E
    res.write(logEntry);
    res.end();
  });
}

//#A Get the temperature and start the chain of calls
//#B A named temperature function
//#C Once the callback for temperature has been called we proceed with calling the getLight function
//#D We then call the named function to log values
//#E Return to the client