Understanding promises in javascript using when library

Question

I've been trying to wrap my head around asynchronous programming and the use of promises. To help understand them, I've written some trivial nested code, but have run into a snag.

Here's the code: http://pastebin.com/hBtk9vER Make sure you install when library (npm install)

var when = require('when');

function promise() {
  console.log("Promise");
    promiseRead().then(function(string) {
    console.log("Inner promise");
    console.log(string);
    });
}

function promiseRead() {
  console.log("PromiseRead");
  return baz().then(function() {
    console.log("Inner Read");
    var deferred = when.defer();
    setTimeout(function() {
      deferred.resolve("Hello World");
    }, 5000);
  });
}

function baz() {
  console.log("BAZ");
  return when(true); 
}

promise();

My problem is that console.log(string) is undefined, when I was expecting it to be "Hello World" after promiseRead() resolves. Interestingly, when I remove the timeout, it works as expected. Can anybody help explain this, why the promise function is executing it's code before promiseRead() has finished timeout.

Much appreciated

I do not have time for a full answer, but in your `Inner Read` you don't return anything (no promise) so your `var deferred = when.defer();` at this place is useless. — t.niese, Jan 16 '14 at 12:11
To add to that, I know you're experimenting, but promises are not much help if you don't use them in a structured way. The point of promises (IMO) is to organize async code linearly. You could make this even more convoluted by adding rejections and passing deferreds around, but I hope you can see now why it's important to keep promise-based code clean. — acjay, Jan 16 '14 at 14:10

score 2 · Accepted Answer · answered Jan 16 '14 at 12:14

2

It looks like you need to return your deferred object in promiseRead()

answered Jan 16 '14 at 12:14

Jozef Dransfield

524
4
11

Awesome, thanks. I was under the impression that `deferred.resolve()` would return a deferred object, but than obviously didn't think about the asynchronous nature of the language. So in my example, was `promiseRead` returning null, before the deferred resolved and the function above it on the stack, had no way of knowing that a promise had to be waiting? – b1nd Jan 17 '14 at 00:28

score 1 · Answer 2 · answered Jan 16 '14 at 12:32

some updates

var when = require('when');

function promise() {
  console.log("Promise");
    promiseRead().then(function(string) {
    console.log("Inner promise");
    console.log(string);
    });
}

function promiseRead() {
  console.log("PromiseRead");
  return baz().then(function() {
    console.log("Inner Read");
    var deferred = when.defer();
    setTimeout(function() {
      deferred.resolve("Hello World");
    }, 5000);
    return deferred.promise;
  });
}

function baz() {
  console.log("BAZ");
  return when(true); 
}

promise();

Is this an answer to your question? Is there still a problem within it? If it is an answer to your question then take the time to explain how you solved it. — t.niese, Jan 16 '14 at 19:22

Understanding promises in javascript using when library

2 Answers2