node.js - Returning a value from an asynchronous recursive function -

i have complex recursive function in node.js, sake of question simplified following:

function sum(tree) {     if (!tree) return 0;     var sumleft = sum(tree.left);     var sumright = sum(tree.right);     return sumleft + tree.val + sumright; }  var exampletree = {     val: 3,     right: { val: 4 },     left: { val: 5,          right: {val: 6},         left: {val: 7}     } }  console.log(sum(exampletree)); // returns 25 = 3+4+5+6+7 

now want convert function "sum" asynchronous function:

function sumasync(tree, callback) {     // ??? } 

but, since function doesn't return value, don't know how values sumright , sumleft , combine them.

one possible option totally rewrite algorithm such iterative , not recursive (as suggested in question: how make synchronous recursive function asynchronous). but, in many cases might complicated , make entire program unreadable. there solution keeps simple structure of recursive function, while making asynchronous?

note: problem not sum values in tree... tree-sum function minimal working example.

edit: based on vkurchatkin's answer , comment, decided use async.parallel. result:

var async = require("async");   function sumasync (tree, callback) {     if (!tree) return setimmediate(callback.bind(null, null, 0));      async.parallel([       sumasync.bind(this, tree.left),       sumasync.bind(this, tree.right),     ],     function(err, results) {         callback(err, tree.val+results[0]+results[1]);     }); }  sumasync(exampletree, function(err, result) {     console.log(result);  // prints 25 }); 

something might work:

function sumasync (tree, callback) {     if (!tree) return setimmediate(callback.bind(null, null, 0));      var pending = 3;     var sum = 0;     var done = false;      function handleerror (err) {        if (!done) {           callback(err);           done = true;         }     }      function _callback (err, res) {       if (err) return handleerror(err);        sum += res;        if (!--pending && !done) {         done = true;         callback(null, sum);       }     }      tree.fetchleft(function (err, left) {       if (err) return handleerror(err);        sumasync(left, _callback);     });      tree.fetchright(function (err, right) {       if (err) return handleerror(err);        sumasync(right, _callback);     });      tree.fetchvalue(_callback); } 

it might complex, it's ad-hoc async.parallel implementation, can use instead.