Wait for request to finish Node.js

Question

I am currently having an issue with figuring out how to wait for the request to finish before returning any data. I do not believe I can do this with a Callback and I have not been able to figure out a good way of using the EventEmitter to do it. The reason I cannot use a callback is because my flow currently works like this.

Request comes into server > Generate XML > Contact remote API for details to finish generating XML > Finish Generating XML > Return request to client

The code I currently have looks very similar to the code included below.

Web Server:

var xml = require('./XMLGenerator');
response.writeHead(200, {'Content-Type': 'text/xml'});
response.write(xml.generateXML());
response.end();

XML Generator:

function generateXML(){
// Code to generate XML
var API = require('./API');
var response = API.getItems("5");
for(var i = 1; i <= response.length; i++)
{
  // more code to generate further XML using the API response
}

// Finish generating and return the XML
}

API Grabber:

function getItems(sort_by, amount) {
var request = require("request")

var url = "https://url.com/api/get_items.json?amount=" + amount;
request({
    url: url,
    json: true
}, function (error, response, body) {
    console.log('we got here!');
   if (!error && response.statusCode === 200) {
        var items = body.data.items;
        console.log(items);

        return items;
    } else {
        console.log("Error connecting to the API: " + url);
        return;
    }
})
}

When running the code and testing directly it returns "undefined" meaning that the request has not been made yet. I just need to know a way to make the XML generator wait for the request to finish before continuing on with the generation. (there may be minor errors in the psudeo code I typed up as it is not an exact copy paste from the source, it does however work in this flow)

Am I just using bad practices, or is this the correct way that I should be attempting this?

EDIT: The problem is not loading the module/API file, that loads perfectly fine. The problem is that the request takes about 2 seconds to complete, and that node moves on before the request completes.

Answer 1

You need to use callbacks. Change your API grabber to this:

function getItems(amount, callback) {
// some code... 
request({
  url: url,
  json: true
}, function (error, response, body) {
   // some code...
   if (!error && response.statusCode === 200) {
      // some code    
      callback(items); <-- pass items to the callback to "return" it
   }
})
}

Then change the xml generator to also accept callbacks:

function generateXML(callback){
// Code to generate XML
var API = require('./API');
API.getItems("5",function(response){
  for(var i = 1; i <= response.length; i++)
  {
    // more code to generate further XML using the API response
  }

  // Finish generating and return the XML
  callback(xml_result); // <-- again, "return" the result via callback
});
}

Then in your server code do:

var xml = require('./XMLGenerator');
response.writeHead(200, {'Content-Type': 'text/xml'});
xml.generateXML(function(xmlstring){
    response.write(xmlstring);
    response.end();
});

Answer 2

Do these changes since request is async, use callbacks.

API.getItems("5", function(rs){
      var response = rs;
      for(var i = 1; i <= response.length; i++)
      {
      // more code to generate further XML using the API response
      }

      // Finish generating and return the XML
    }

});


...
function getItems(sort_by, amount, callback) {...

...
callback(items); //Instead of return items;

...

You cannot return from an async call, which is request module in your case. In such cases you can either use promises or callbacks. This is a solution with callback.

The actual problem of it returning undefined is it doesnot wait for var response = API.getItems("5"); to execute completely and executes the next line and hence you get response as undefined. I hope you get the point.\

Also I hope

response.writeHead(200, {'Content-Type': 'text/xml'});
response.write(xml.generateXML());
response.end();

is somewhere inside some callback of an API or http.createServer.