Better pattern for accessing nested data in Javascript

Sorry to say but you're a little late. // some code was not async.

Diego Pamio Over a year ago

ouch, that's because I took to much time to prettify the answer, lol.

Don't worry, it happens.

@PatrickRoberts at least he added a postscript saying this is only necessary if there's async methods in the inner loop.

Patrick Roberts · Accepted Answer · 2015-06-17 04:07:00Z

0

You don't need to keep track of the last item. Just call the callback after both loops exit. I also changed the .forEach to for loops as these execute faster.

function countSuccessPosts(rssFeeds, cb) {
  var index, itemIndex, feed, item;

  for (index = 0; index < rssFeeds.length; index++) {
    feed = rssFeeds[index];

    for (itemIndex = 0; itemIndex < feed.itemsToPost.length; itemIndex++) {
      item = feed.itemsToPost[itemIndex];

      if(item.response && item.response.id) {
        successCounter++;
      }
    }
  }

  cb(null, rssFeeds, successCounter);
}

Of course, if you'd rather call countSuccessPosts without a callback the calling code can look like:

var successPosts = countSuccessPosts(rssFeeds);

And you can reformat the function to look like this:

function countSuccessPosts(rssFeeds) {
  var index, itemIndex, feed, item, successCounter = 0;

  for (index = 0; index < rssFeeds.length; index++) {
    feed = rssFeeds[index];

    for (itemIndex = 0; itemIndex < feed.itemsToPost.length; itemIndex++) {
      item = feed.itemsToPost[itemIndex];

      if(item.response && item.response.id) {
        successCounter++;
      }
    }
  }

  return successCounter;
}

edited Jun 17, 2015 at 4:07

answered Jun 17, 2015 at 3:45

Patrick Roberts

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7 Comments

A) There's no need to use a callback when rssFeeds can be accessed synchronously. B) Otherwise, If // some code has async calls, your code breaks anyway because cb is called before the async function would finish.

@naomik it is possible he's using async simply because the syntax helps organize and simplify his already pyramid-like code.

Well it's bad to write async control around synchronous code. Don't encourage this. JavaScript is single-threaded. Adding a callback to your function doesn't change this.

Yeah that's basically it. I'm just looking for a better pattern for looping through this data, since I have to do it relatively often, as I'm passing this array through the waterfall pattern and mutating it along the way.

@mrmayfield I added a quick edit to ensure that item.response.id doesn't throw a reference error if item.response is undefined.

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Mulan · Accepted Answer · 2015-06-17 04:17:55Z

0

Wait, why are you using a callback when you can read the data synchronously?

After you've updated your question, it looks like you're just summing a number of elements in an array

Here's a fully synchronous version that counts the number of itemsToPost that have a valid response.id set.

function countSuccessPosts(rssFeeds) {
  return rssFeeds.reduce(function(sum, x) {
    return sum + x.itemsToPost.filter(function(y) {
      return !!y.response.id;
    }).length;
  }, 0);
}

If you're required to inject this into an async control flow, you can easily put a wrapper on it

function(rssFeeds, done) {
  done(null, rssFeeds, countSuccessPosts(rssFeeds));
}

The point tho, is that countSuccessPosts has a synchronous API because everything that happens within that function is synchronous.

edited Jun 17, 2015 at 4:17

answered Jun 17, 2015 at 4:00

Mulan

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5 Comments

Sure your code is shorter, but it runs slower than my solution, and is harder to read.

The reason is that I'm splitting the functionality out into small methods, many of which rely on async (database and network API calls), and organizing the code using the async waterfall pattern and passing through the rssFeeds array.

Hey, I'm just looking to learn. I wouldn't doubt it at all if my design was unoptimized. If you have a suggestion for how to better organize my code given my explanation above of my needs I would be happy to hear it!

@mrmayfield I provided the synchronous function and the async wrapper to use in your waterfall control code. Think about it tho: just because you're counting the posts in an async portion of code, doesn't mean that the count is async. What happens if you want to call the count function in a different part of your code? With the async-only api, it would seem kind of broken because it's forcing the callback even tho it's completely synchronous. Keep the functions intent separate from the control flow and just use the wrapper where necessary.