node.js - Javascript asynchronous calls chaining -

given array contains objects of type , b, b can transformed set of type objects, via asynchronous call, best way of transforming array objects array (transforming each b object in set of corresponding objects) , execute callback when bs transformed?

    list = [a, b, a, b, a, a, b, a];      function transform (b, callback) {        //transform b type object type objects array [a, a, a].....        callback([a, a, a]);     }      transformbobjectsintoaobjects(list, callback) {        // ?????????       callback(list); // should type objects     } 

well, need execute final callbacks after callbacks transformbtolist have returned result. there multiple ways it:

1. count how many callbacks have passed away , decrement when call back, , know you're finished when counter has reached 0 again.

   however, cumbersome, , there libraries it:

2. async.js well-known , easy use:

   function transform(b, callback) { … } function transformbobjectsintoaobjects(list, callback) {   async.map(list, function(x, cb) {     if (x b)       transform(x, cb)     else       cb([x])   }, function(results) {     callback(concat(results))   } } 

3. promises (there many implementations superior approach. might bit more complex understand, have nice properties , lead nice & concise syntax. in case, be

   function transform(b) { // no callback!   // async:     resolve([a, a, a]); // see docs of promise library   return promise;       // exact reference } function transformbobjectsintoaobjects(list) {   return promise.all(list.map(function(x) {     return (x b) ? transform(x) : promise.resolve([x]);   })).then(concat); }