Recursive Javascript flow control.
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Flatify is a simple, recursive and powerful dynamic flow management library for asynchronous Javascript and Node.JS. It was built to make callback heavy code, modular easy to read, write and maintain; in other words, to avoid going to Callback Hell. Compared to the popular Async library, flatify has much more advanced fine tuning for concurrency features, but none of Async's collections and other utilities.

I wrote this as a proof of concept and to gain a deeper understanding of asynchronous flow management. Streamline is the tool I recommend to solve the async problem in Node.JS, it's a really amazing piece of software.

Basic features

Flatify works by building a queue of ordered jobs. Suppose you have 3 HTTP calls to make, but each call needs the result of the previous one.


var callA = function(error, callback){
  makeHTTPcall( ... , function(result){
		callback(null, result);
var callB = function(error, resultA, callback){ ... };
var callC = function(error, resultB, callback){ ... };

new flatify(this).seq(callA).seq(callB).seq(callC).run(function(error, resultC){
	//do something with resultC

The seq() method adds a job to the end of the jobs queue. Once run() is called, every job in the queue is run in a sequential order. But now suppose that callA and callB can be made in parallel and that callC can only be made once both callA and callB have completed:


var callA = function(error, callback){ ... }
var callB = function(error, callback){ ... }
var callC = function(error, results, callback){ ... }

new flatify(this).par([callA, callB]).seq(callC).run(function(error, resultC){
	//do something with resultC

Flatify makes it easy to organize asynchronous code in a simple and beautiful way, but the real power of flatify is documented in the Advanced Features section.


Node.JS Simply place the flatify.js file in your project folder and include it:

var flatify = require(__dirname + "/flatify.js").flatify;

Browser Just import it!

<script src="flatify.js" type="text/javascript"></script>


Every method that doesn't return a value returns "this" to make calls chainable.

.seq(job, options, index)

Options and index are optional. Add the job at the specified index in the jobs queue, end of the queue by default.

Job function that conforms to Node.JS's standard arguments format: function(error, param1, param2, param3, ..., callback)

Options: object. Key:

  • "cont" (boolean). If set to true, the next job in the job queue will be called even if the current job returned an error. By default, returning an error will cause flatify to jump to the run() callback.

Index Index in the job queue to add. By default, the job will be added to the end of the queue.

.par(jobs, options, index)

Options and index are optional. Add multiple jobs to be executed in parallel. The next job in the queue will be called once all these parallel jobs have completed. If one of them returns an error, the next job in the queue is called immediately unless the "wait" option is set to true.

Jobs: An array of jobs OR a single function (not in an array). For the latter, make sure to specify the "nb" option.

Options: object. Keys:

  • "cont" (boolean). Same as for seq().
  • "nb" (integer). If Jobs is a single function, "nb" is the number of times it must be run in parallel. Default is 1.
  • "wait" (boolean). If set to true and one of the parallel jobs returns an error, flatify will wait for all jobs to complete before continuing.

Index: Same as previously.


Flatify starts going through the job queue.

Job: this is the final job, it will be called once flatify has reached the end of the queue. It is mandatory.


Returns the current job queue index.


Sets the job queue index. After the current job finishes, the queue will continue at that index. Warning: it's easy to create infinite loops by backtracking without checks. Read the Advanced Features section for more information.


Returns the number of jobs in the job queue.


Removes the job at the specified index. To add a job at a specific index, use the seq() and par() methods. Read the Advanced Features section for more information.


Returns the internal execution context. Read the Advanced Features section for more information.


The current job will finish, but the next one won't be called. If the curent job was the last one in the queue, the "exit job" (the one supplied to the run() method) will be called.


Resumes execution after calling pause().


Returns true if run() has been called, false otherwise.


Returns true if pause() has been called, false otherwise.


Returns true if the "exit job" (the run() one) has been called.

Advanced features

Default options

The default "cont" and "wait" options (false and true, respectively) can be overriden

new flatify(this, {"cont":true, "wait":false}).par(
	function(){ ... } //Here "cont" is true and "wait" is false
	function(){ ... }, {"cont":false} //Here both "cont" and "wait" are false.
).run( ... );


Suppose you want to make 2 HTTP calls sequentially: callA, then callB.

new flatify(this).seq(callA).seq(callB).run(function(error, result){
	//do something with result

... except that you want to do the above 4 times in parallel:

new flatify(this).par(function(error, callback){
	new flatify(this).seq(callA).seq(callB).run(function(error, result){
		callback(null, result);
}, {"nb":4}).run(function(error, results){
	//Here results is an array containing 4 results,
	//1 from each individual parallel job (each one made from 2 sequential jobs)

By embedding flatify objects recursively, it becomes possible to easily design complex operations.

Shared context

A flatify object is made of only one context, which allows parallel jobs to share data. In other words, at any point in the jobs queue, the 'this' object refers to the same unique object.

new flatify(this).seq(function(error, callback){
	this.test1 = "ABC";
	this.test2 = [];
	this.test3 = 1;
}).par(function(error, callback){
}, {"nb":5}).run(function(error){
	console.log(this.test1); //Outputs "ABC"
	console.log(this.test2); //Outputs [1,2,3,4,5]

It also works recursively!

new flatify(this).seq(function(error, callback){
	this.test1 = "ABC";
	new flatify(this).seq(function(error, callback){
		console.log(this.test1); //Outputs "ABC"
		this.test2 = "DEF";
	console.log(this.test1); //Outputs "ABC"
	console.log(this.test2); //Outputs "DEF"


The flatify object can be accessed and modified during execution. The shared context contains a "flatify" object with the following structure:

this.flatify object

  • currentInstance. Refers to the flatify instance that is currently running.
  • masterInstance. Refers to the TOP flatify instance. It is the same instance as currentInstance unless the currentInstance is embedded within a flatify instance.

Any flatify instance

Apart from the methods described in the Methods section, a flatify instance also contains the following properties:

  • level. Depth level. The top flatify instance has a level of 0. An instance within an instance within an instance has a level of 2.
  • parentInstance. A reference to the instance one level higher, null if the currentInstance is the top instance.
  • defaultOptions. See the Default Options section for more information
  • cont. The "cont" setting for the current job.
  • wait. The "wait" setting for the current job.

The methods and properties beginning with an underscore are for internal use.


new flatify(this, {"cont":true}).seq(function(error, callback){
	callback("error 1");
}).seq(function(error, callback){
	callback("error 2");
}).seq(function(error, callback){
	console.log(error); //Outputs "error 1"
	callback("error 3");
	console.log(error); //Ouputs "error 3"
//The second job was never called because it was deleted by the first job.
new flatify(this).seq(function(error, callback){
	if (!this.secondPass){
		this.secondPass = false;
		this.nbSeq = 0;
}).seq(function(error, callback){
}).seq(function(error, callback){
	if (!this.secondPass){
		this.secondPass = true;
		this.flatify.currentInstance.setNextIndex(0); //Goes back to the first job, but now this.secondPass is true
	console.log(this.nbSeq); //Outputs 6

More examples can be found in the tests/testflatify.js file.