Bigood Bigood - 1 year ago 82
Javascript Question

Return predicate from nested async function call

I need to filter an array, depending whether or not each element's sub-element, obtained by retrieving data from an external API, matches a criteria.

The code might be clearer than an explanation :

//Suppose there is an array filled
//_.filter(array, predicate)
var filtered_array = _.filter(array,function(element){
//Async operation
webservices.doStuff(element, function(sub_element){
//This return is meant for the filter, but returns on .doStuff callback
return sub_element.thing === "ok";

The problem here, is I don't know how to return the predicate's result here, as
is asynchronous!

Any idea?

Answer Source

I would use Q or async to do this. Consider following example, you wrap each web service call in a promise and define another promise that will be resolved when all promises are resolved. Then you filter results ( ie array of all returns from all calls) according to your logic.

 var source = [{a : 1}, {a:2}...] // your 

 var all = Q.all( 
               => Q((success, error) 
                  =>, success, error))))

 all.then(function(ret) {
     return filtered = _.filter(ret, function (data) {
              return data.thing === "ok"; 
 }).then(function(filtered) {

    // do something with filtered


Most likely your web service proxy already have binding with Q or another promise library. So your code is going to be simpler, something like:

var all = Q.all(


Or at least you can refactor it to be that simple.

Overall due to non-blocking-accept-callback-and-go nature of node.js environment, it is essential to master one promise library or another, async and Q are two most popular to the best of my knowledge.

(Update: right now it seems to be bluebird, and it is fastest too.)

Having either of them allow you to come up with much easier design when dealing with async calls.

Recommended from our users: Dynamic Network Monitoring from WhatsUp Gold from IPSwitch. Free Download