Node.JS/async for loop, current iteration in nested callback

Question

trying to wrap my head around some basic async programming concepts.

Right now I essentially have two functions, for now lets call them getOne and getAll. getAll essentially loops of a series and makes a call to getOne, then maps it to a hash and sends it back down the chain. Here is basically what I have, in coffeescript:

getOne = (key, callback) ->
    try
        # find object referenced by key
        object = "blah blah"
        callback(null, object)
    catch(error)
        callback(error, null)

getAll = (keys, callback) ->
    results = {}
    try
        count = keys.length
        for i, key of keys
            count--
            getOne key, (err, result) ->
                if(err)
                   # do something
                else
                   results[key] = result
                if count is 0
                   callback "", results
    catch
        callback error, results

The above code essentially outputs a hash where the keys don't necessarily lineup with the expected data, which is expected the way a for loop is async processed. However, I can't seem to come up with a solution that doesn't block the loop and cause everything to be processed in a series instead of maintaining parallel processing.

What am I missing?

Waylon Flinn · Accepted Answer · 2012-07-23 19:32:58Z

3

There isn't anything inherently asynchronous about the code you've written. If you're worried about the callbacks in getOne taking a long time to execute, you have a couple of options.

use the async library
use process.nextTick to defer execution of each iteration of your for loop

The first option is definitely easier.

The second option involves rewriting your getAll function as a recursive function (instead of using for) and wrapping recursive subcalls in process.nextTick.

This article is reasonably useful for understanding how to use process.nextTick to accomplish this: Understanding process.nextTick()

edited Jul 23, 2012 at 19:32

answered Jul 23, 2012 at 19:05

Waylon Flinn

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Jonathan Ong · Accepted Answer · 2012-07-23 19:37:42Z

2

Using the async library, it's as simple as this:

getAll = (keys, callback) ->
  funcs = {}
  keys.forEach (key) => funcs[key] = (_callback) =>
    getOne key, _callback
  async.parallel funcs, callback

answered Jul 23, 2012 at 19:37

Jonathan Ong

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