Maybe I just don’t get it yet, but it seems like the interest around “DOM Futures” (e.g. promises) is growing, and is becoming a proposed spec for future javascript engines. What DOM Futures is is another way to handle asynchronous flow in your javascript apps. Currently you can use nested callbacks, which are probably the most common, or you can use custom events (like Backbone, jQuery, or any other publish/subscribe framework).

While I see that DOM Futures / Promises could cut down on lines of code, I’m not sure what else it truly gives me as a developer. But, I may not fully grok it yet.

I’ve put together two examples of a ficticious javascript app that needs to get a user’s posts via ajax. Below you’ll see each example via DOM Futures and via custom events.

What am I missing?

DOM Futures

From the WHATWG spec. This example makes a JSON ajax request, and then returns a promise object that has the “.done()” method on it, that will asynchronously call one of two callbacks depending on the response of the json call.

fetchJSON("/user/posts").done(showPosts, showFailcat)

function fetchJSON(url){
    // create json request, wrap in a promiseObject
    return promiseObject;
}

function postsSuccess(data){
}

function showPosts(data) {
    postsSuccess(data);
    //...
}

function showFailcat(error){
    //...
}

Custom Events

A similar example using some custom event framework, that attaches/subscribes using “on()” and triggers/publishes to an event using “trigger()”.

$.getJSON("/usr/posts",onPostsSuccess,onPostsError)

function onPostsSuccess(data){
    // let's just say we have two events we want fired
    trigger("posts:success",data);
    trigger("posts:show",data);
}

function onPostsError(error) { trigger("posts:error",error); }

function postsSuccess(posts){
}

function postsShow(posts){
}

on("posts:success",postsSuccss);
on("posts:success",postsShow);

function postsError(error){
}
on("posts:error", postsError);