javascript - Asynchronous code and loops within Node.js? -

i've done hours of research on asynchronous programming, can't seem grasp 1 single concept within node, wondering if here me.

i've written following code sample return / output simple string concatenation of strings object:

var itemcollection = {     item1 : [{ foo : "bar" }, { foo : "bar" }, { foo : "bar" }],     item2 : [{ foo : "bar" }, { foo : "bar" }, { foo : "bar" }],     item3 : [{ foo : "bar" }, { foo : "bar" }, { foo : "bar" }] }  var astring = "";  for(item in itemcollection){     (var = 0; < itemcollection[item].length; i++) {         var anitem = itemcollection[item][i];          //somefunctionthatdoesalongiooperation takes item param, plus callback.         somefunctionthatdoesalongiooperation(anitem, function(databackfromthisfunction){             // data returned function             astring += databackfromthisfunction.datatoappend;         });     }; }  console.log(astring); 

so understand, languages other javascript run somefunctionthatdoesalongiooperation synchronously , script run in 'blocking mode'. mean value astring returned / outputted correct value.

however, node runs asynchronously, code can continue run @ anytime , tasks may not complete in order. because of way event loop works in node. think this.

so question comes in. if wanted value astring returned / outputted correct value in other languages, need loops within code example? or put question in more technical words: correct approach making astring return expected result, io operations (which take longer amount of time run) aren't completed after script has finished executing when astring has been returned?

i hope question makes sense, if doesn't, please let me know , make edits appropriate.

thank you

since function apply each item asynchronous, loop processes them must asynchronous (likewise function consumes result of loop must async). check out bob nystrom's "what color function?" more insight on particular point.

there's 2 ways (both using caolan's async library wrap nasty callback logic):

• do 1 async operation 1 @ time, waiting previous finish before next can begin. similar way traditional synchronous loop runs. can async.reduce:

  async.reduce(itemcollection, "", function(memo, item, callback) {     somefunctionthatdoesalongiooperation(item, function(databackfromthisfunction) {         callback(null, memo + databackfromthisfunction.datatoappend);     }); }, function(err, result) {     var astring = result; }); 

• of course, there's little point in having async code if don't reap it's benefits , execute many things @ once. can async operations in parallel , reduce @ once in single step afterwards. i've found great if processing each item requires long operation such network i/o, since can kick off , wait many requests @ once. use async.map achieve this:

  async.map(itemcollection, function(item, cb) {     somefunctionthatdoesalongiooperation(item, function(databackfromthisfunction) {         cb(null, databackfromthisfunction.datatoappend);     }); }, function(err, results) {     var astring = results.join(''); });