Assing variable from inner callback

Question

In my honest opinion this question is not duplicated because I know how to fix it using a callback as explain in the linked question, this question is specific about a situation where I cannot modify the outfer function and this is not covered in the another question

I use a library that I cannot modify, the library do something like this:

var result = Foo()

Then it uses result for the work. I can not modify how result works, I can only modify Foo because I pass it as parameter to the library.

If foo has something like this

funcion Foo(){
  return "Hello"
}

It works perfect. Result is Hello and it works.

The issue is that my Foo has an async call inside it. It calls a web server and then it should return the response of the server. So

funcion Foo(){
   request.get('http://server.com', function(error, response, body){
      var parsed = JSON.parse(body);
      return parsed.Hello;
   });
}

In this case when var result = Foo();, result does not contains the response of the server (because is async) and continue with an empty result.

So if I only can modify Foo what should I do?

Update: If you dont find it possible I would love to hear alternatives like what should I ask in PR for the library, or a way to force the request to be syncronous

*>what should I ask in PR for the library* It would help if you described the module in more details. — lleaff, Dec 29 '15 at 02:28
@SteveHansell I dont think so... This is where the library do that: (new_buf is result and filter is Foo in my explanation) https://github.com/chazomaticus/Haraka/blob/ece98e4797d78af9aad98c0469afde50bb9ba38d/mailbody.js#L255 — Ricardo Polo Jaramillo, Dec 29 '15 at 02:34

score 1 · Accepted Answer · answered Dec 29 '15 at 02:28

1

This is how you can work with a synchronous request:

funcion Foo(){
   var request = new XMLHttpRequest();
   request.open("GET", "http://server.com", false);
   request.send();
   function handleResponse(response) {
      console.log(response);
   }
   handleResponse(request.responseText);
}

If you can only modify Foo, then modify it to send the request synchronously.

answered Dec 29 '15 at 02:28

Lajos Arpad

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two questions: 1. This is a Node.js app. Using sync calls will lock all execution of all users using it, or only the execution of the current user? If is only the current user I think I can accept it. 2. In Node.js not exist XMLHttpRequest so should i use this port https://github.com/driverdan/node-XMLHttpRequest or are a better way to sync request in Node.js? – Ricardo Polo Jaramillo Dec 29 '15 at 02:43
@RicardoPolo, 1. It should not block everyone, but it is safer to test it 2. Take a look at this library: https://github.com/coolaj86/futures – Lajos Arpad Dec 29 '15 at 03:20
thank you! I will go with your option. I open another question to expand what I just comment to you http://stackoverflow.com/questions/34503840/using-a-blocking-long-sync-operation-will-pause-all-users-in-node-js :) – Ricardo Polo Jaramillo Dec 29 '15 at 03:34

lleaff · Answer 2 · 2015-12-29T02:27:06.707

0

What you're trying to do is impossible per se, once code is asynchronous, it can't be used as you would synchronous code. That's why callback hell is a thing.

You could try to analyze the module internals and monkey patch it, but that may not be possible depending on what exports the module exposes.

edited Dec 29 '15 at 02:27

answered Dec 29 '15 at 02:17

lleaff

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In this case, the solution is to modify Foo to send the request synchronously. Not a nice thing to do, but that is the solution, since the code using Foo expects Foo to work synchronously. – Lajos Arpad Dec 29 '15 at 02:33
You are completely right, I totally forgot XMLHttpRequest had that option. – lleaff Dec 29 '15 at 02:42

Assing variable from inner callback

2 Answers2

Linked