With node.js how to I set a var to response from HTTP client?

I know this is probably Asynchronous Javascript 101 and I do have some books on the Kindle I could consult, but I am nowhere near my device.

I have a node app with a variable being assigned to a module that I'm loading. The module has one function that downloads a string of data from a URL.

The problem is, how do I not set the variable until the request has returned?

My code looks like this:

Downloader.js:

var http = require('http');
exports.downloadString = function(str) {
  console.log("Downloading from " + str);
  http.get(str, function(res) {
    var data = [];
    console.log("Got response: " + res.statusCode);
    res.on('data', function (chunk) {
      data.push(chunk);
    });
    res.on('end', function() {
      return data.join('');
    });
  }).on('error', function(e) {
    console.log("Got error: " + e.message);
  });
}

app.js:

var downloader = require('./lib/Downloader')
, dateParser = require('./lib/DateParser')
, eventIdGenerator = require('./lib/EventIdGenerator');

var report = downloader.downloadString("http://exampleapi.com");

console.log(report);

I need to wait until the variable named "report" is populated.

Obviously this means my Mocha tests are also failing as I'm still unsure of how to tell the test to wait until the variable is filled.

I'm sure this is extremely simple, but I am drawing a blank!

Thanks!

Node.js is (mostly) asynchronous, so you'd need to pass a callback function to your module:

Downloader.js:

var http = require('http');
exports.downloadString = function(str, callback) {
  console.log("Downloading from " + str);
  http.get(str, function(res) {
    var data = [];
    console.log("Got response: " + res.statusCode);
    res.on('data', function (chunk) {
      data.push(chunk);
    });
    res.on('end', function() {
      callback(data.join(''));
    });
  }).on('error', function(e) {
    console.log("Got error: " + e.message);
  });
};

app.js:

var downloader = require('./lib/Downloader')
, dateParser = require('./lib/DateParser')
, eventIdGenerator = require('./lib/EventIdGenerator');

downloader.downloadString("http://exampleapi.com", function(report) {
    console.log(report);
});