Sequential programming for node.js, end of callback hell / pyramid of doom





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Sequential programming for node.js, end of callback hell.

Simple, straightforward abstraction over Fibers.

By using wait.for, you can call any nodejs standard async function in sequential/Sync mode, waiting for result data, without blocking node's event loop (thanks to fibers)

A nodejs standard async function is a function in which the last parameter is a callback: function(err,data)


  • Avoid callback hell / pyramid of doom
  • Simpler, sequential programming when required, without blocking node's event loop (thanks to fibers)
  • Simpler, try-catch exception programming. (default callback handler is: if (err) throw err; else return data)
  • You can also launch multiple parallel non-concurrent fibers.
  • No multi-threaded debugging nightmares, only one fiber running at a given time (thanks to fibers)
  • Can use any node-standard async function with callback(err,data) as last parameter.
  • Plays along with node programming style. Write your async functions with callback(err,data), but use them in sequential/SYNC mode when required.
  • Plays along with node cluster. You design for one thread/processor, then scale with cluster on multicores.


###Aug-2013 - Wait.for-ES6 based on ES6-generators

I've developed a version based on JavaScript upcoming ES6-Harmony generators. It's not based on node-fibers. Surprisingly, ES6 based implementation of wait.for(asyncFn) is almost a no-op, you can even completely omit it. Warning: Bleeding edge. Check [Wait.for-ES6] (


npm install wait.for

Proper Use:

You need to be in a Fiber to be able to use wait.for. The ideal place to launch a fiber is when a request arrives, to handle it:

var server = http.createServer(
  function(req, res){
    wait.launchFiber(handler,req,res); //handle in a fiber, keep node spinning

then,at function handler(req,res) and every function you call from there, you'll be able to use wait.for(ayncFn...

Minimal running example

var wait = require('wait.for');

function anyStandardAsync(param, callback){
    setTimeout( function(){
                  callback(null,'hi '+param);
        }, 5000);

function  testFunction(){
    console.log('fiber start');
    var result = wait.for(anyStandardAsync,'test');
    console.log('function returned:', result);
    console.log('fiber end');

console.log('app start');
console.log('after launch');

Basic Usage Example with Express.js

var wait = require('wait.for');
var express = require('express');
var app = express();

// in  a Fiber
function handleGet(req, res){
  res.send( wait.for(fs.readFile,'largeFile.html') );

app.get('/', function(req,res){
  wait.launchFiber(handleGet, req, res); //handle in a fiber, keep node spinning


Cradle/couchdb Usage Example

see cradle example

Generic Usage:

var wait=require('wait.for');

// launch a new fiber
wait.launchFiber(my_sequential_function, arg,arg,...)

// in a fiber.. We can wait for async functions
function my_sequential_function(arg,arg...){
    // call async_function(arg1), wait for result, return data
    var myObj = wait.for(async_function, arg1); 
    // call myObj.querydata(arg1,arg2), wait for result, return data
    var myObjData = wait.forMethod(myObj,'queryData', arg1, arg2);

##Notes on non-standard callbacks. e.g.: connection.query from mysql, database.prepare on node-sqlite3

wait.for expects standardized callbacks. A standardized callback always returns (err,data) in that order.

A solution for the sql.query method and other non-standard callbacks is to create a wrapper function standardizing the callback, e.g.:

     connection.prototype.q = function(sql, params, stdCallback){ 
                 this.query(sql,params, function(err,rows,columns){ 
                                     return stdCallback(err,{rows:rows,columns:columns}); 


  try {
  var result = wait.forMethod(connection, "q", options.sql, options.params); 
catch(err) {

e.g.: node-sqlite3's database.prepare

var sqlite3 = require('sqlite3').verbose();
var db = new sqlite3.Database(':memory:');

db.prototype.prep = function(sql, stdCallback){ 
             var stmt = this.prepare(sql, function(err){ 
                                 return stdCallback(err, stmt); 

var stmt = wait.forMethod (db, 'prep', "INSERT OR REPLACE INTO foo (a,b,c) VALUES (?,?,?)");

More Examples:

DNS testing, using pure node.js (a little of callback hell):

var dns = require("dns");
function test(){ 
    dns.resolve4("", function(err, addresses) {
        if (err) throw err;
        for (var i = 0; i < addresses.length; i++) {
            var a = addresses[i];
            dns.reverse(a, function (err, data) {
                if (err) throw err;
                console.log("reverse for " + a + ": " + JSON.stringify(data));


THE SAME CODE, using wait.for (sequential):

var dns = require("dns"), wait=require('wait.for');

function test(){
    var addresses = wait.for(dns.resolve4,"");
    for (var i = 0; i < addresses.length; i++) {
        var a = addresses[i];
        console.log("reverse for " + a + ": " + JSON.stringify(wait.for(dns.reverse,a)));


Database example (pseudocode)

using pure node.js (a callback hell):

var db = require("some-db-abstraction");

function handleWithdrawal(req,res){  
    try {
        var amount=req.param("amount");"* from sessions where session_id=?",req.param("session_id"),function(err,sessiondata) {
            if (err) throw err;
  "* from accounts where user_id=?",sessiondata.user_ID),function(err,accountdata) {
                if (err) throw err;
                if (accountdata.balance < amount) throw new Error('insufficient funds');
                db.execute("withdrawal(?,?)",accountdata.ID,req.param("amount"), function(err,data) {
                    if (err) throw err;
                    res.write("withdrawal OK, amount: "+ req.param("amount"));
          "balance from accounts where account_id=?", accountdata.ID,function(err,balance) {
                        if (err) throw err;
                        res.end("your current balance is "  + balance.amount);
    catch(err) {
        res.end("Withdrawal error: "  + err.message);

Note: The above code, although it looks like it will catch the exceptions, it will not. Catching exceptions with callback hell adds a lot of pain, and i'm not sure if you will have the 'res' parameter to respond to the user. If somebody like to fix this example... be my guest.

THE SAME CODE, using wait.for (sequential logic - sequential programming):

var db = require("some-db-abstraction"), wait=require('wait.for');

function handleWithdrawal(req,res){  
    try {
        var amount=req.param("amount");
        sessiondata = wait.forMethod(db,"select","* from session where session_id=?",req.param("session_id"));
        accountdata = wait.forMethod(db,"select","* from accounts where user_id=?",sessiondata.user_ID);
        if (accountdata.balance < amount) throw new Error('insufficient funds');
        res.write("withdrawal OK, amount: "+ req.param("amount"));
        balance = wait.forMethod(db,"select","balance from accounts where account_id=?", accountdata.ID);
        res.end("your current balance is "  + balance.amount);
    catch(err) {
        res.end("Withdrawal error: "  + err.message);

Note: Exceptions will be catched as expected. db methods (, db.execute) will be called with this=db

##How does wait.launchFiber works?

wait.launchFiber(genFn,param1,param2) starts executing the function genFn as a fiber-generator until a "yield" (wait.for) is found, then wait.launchFiber execute the "yielded" value (a call to an async function), and links generator's "next" with the async callback(err,data), so when the async finishes and the callback is called, the fiber/generator "continues" after the var x =wait.for(...).

Parallel Extensions


Note: must be in a Fiber


  • functions: Array = [[func,arg,arg],[func,arg,arg],...]

wait.parallel.launch expects an array of [[func,arg,arg..],[func,arg,arg..],...] and then launches a fiber for each function call, in parallel, and waits for all the fibers to complete.

The functions to be called should not be async functions.

Each called sync function will be executed in it's own fiber, and this sync function should/can use data=wait.for(..) internally in order to call async functions.

####actions: -launchs a fiber for each func -the fiber does resultArray[index] = func.apply(undefined,args)


  • array with a result for each function
  • do not "returns" until all fibers complete
  • throws if error, mappedFn:function)

Note: must be in a Fiber


  • arr: Array
  • mappedFn = function(item,index,arr) -- mappedFn should return converted item. Since we're in a fiber -- mappedFn can use wait.for and also throw/try/catch


  • array with converted items
  • do not "returns" until all fibers complete
  • throws if error

###wait.parallel.filter(arr:Array, itemTestFn:function)

Note: must be in a Fiber


  • arr: Array
  • itemTestFn = function(item,index,arr) -- itemTestFn should return true|false. Since we're in a fiber -- itemTestFn can use wait.for and also throw/try/catch


  • array with items where itemTestFn() returned true
  • do not "returns" until all fibers complete
  • throws if error

Parallel Usage Example: see:

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