aync juuggling - can't explain behaiviour

[swinston100]

This is about Juggling Async in Learnyounode (Exercise 9 of 13)

I have the program working by changing the location of

if(count==3) printResults();

If it's located in the bl function then it works but I move this code outside that function then the count never reaches 3. Can someone please explain to a relative beginner why this is.

The code below contains a console.log() so that I could determine the count as the program ran. This is why I am getting output 0,1,2... :-)

Thanks!

• node version: v4.4.1
• npm version: 2.14.20

  Error output

  1. ACTUAL: "a: 0"
  2. EXPECTED: "As cross as a pint no dramas as cross as a dipstick. Flat out like a sleepout my it' ll be slab. He hasn't got a cactus mate and as busy as a bush telly. "
  3. ACTUAL: "a: 1"
  4. EXPECTED: "As stands out like scratchy with she'll be right dob. Flat out like a ridgy-didge pi ece of piss grab us a cobber. Lets throw a jillaroo bloody stands out like a aussie salute. "
  5. ACTUAL: "a: 2"
  6. EXPECTED: "As stands out like strides also flat out like a aussie rules footy. As cunning as a billy also come a dog's balls. Come a ute heaps he hasn't got a blow in the bag. As stands out like bathers also it'll be vinnie's. As cross as a bradman how gutful of piker. "
  7. ACTUAL: ""
  8. EXPECTED: ""

     My Code

var http = require('http');
var bl = require('bl');
var results = [];
var count = 0;
for (var i = 0; i < 3; i++)
  httpGet(i)

function httpGet(index) {

  http.get(process.argv[index+2], function(response){
    response.pipe(bl(function(err,data){
      results[index] = data.toString();
      count++

      // works here - comment to get working
      //if(count==3){ printResults(); }

    }));

        console.log('count: ' + count);
        // doesn't work here
        if(count==3){ printResults(); }

  });

}

function printResults () {
  for (var i = 0; i < 3; i++)
    console.log(results[i]);
  }

[hackergrrl]

Let's flatten out your httpGet function and its two callback functions into three distinct functions:

function httpGet(index) {
  http.get(process.argv[index+2], handleResponse)
}

function handleResponse(response){
    response.pipe(bl(handleData))
}

function handleData(err,data){
      results[index] = data.toString();
      count++

      // (A)
      if(count==3){ printResults(); }
}

// (B)
if(count==3){ printResults(); }

Where control flows from httpGet -> handleResponse -> handleData each timehttpGet is called.

This is a bit more intuitive to think about -- many nested callbacks can obfuscate the control flow. (B) is actually executed before the code in any of these functions is: http.get() triggers an asynchronous process that will result in handleResponse being called at some point in the future.

[swinston100]

Many thanks @noffle for making my code more understandable and for the great explanation :-) :+1: