Manage async callbacks
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README.md

flow.js

Manage async callbacks.

Aim of this library it to improve readability of async calls by "linearizing" async calls, so that one can read async code from top-to-bottom without losing track of the flow of execution.

Why not use async library? Because it has too much API, and many of them, IMO, are redundant and not helping much in readability. The one that I do like is async.seq() and this library is like async.seq() with few more features.

Why not use Promises? Promises are great, but nodejs/iojs APIs aren't using them yet. Libraries like BlueBird helps you convert nodejs callback APIs to Promises. So you might want to look into that. Ultimately flowjs is another way to deal with callback hell.

With that said, ES7 async/await syntax makes code much more readable than any existing solution. So this library will ultimately become obsolete when ES7 async/await becomes more available for us to use.

Basic API

flow(func1, func2, func3, ...[, scope])(); where each function gets ctl, errors, results as arguments.

The functions are "tasks" that are meant to be executed one after another.

Within a "task", async operations can be executed. And once the async operations complete, the next task should be called. You have access to a control object (ctl) that you can use to call the next task when you are done.

Basic example
flow(function (ctl) { //Task 1 - Simple example. No async stuff here
    console.log('Executing Task 1.');

    //ctl is a control object, that you can use to continue to next task or execute parallel operations.
    ctl.next(null, 'Task 1 result'); //Call ctl.next() to execute next task.
}, function (ctl, err, result) { //Task 2 - Managing two parallel async calls
    console.log(result);

    //Do two async operations in parallel.
    ctl.parallel(['Async 1', 'Async 2'], function (item, callback) {
        //Once both setTimeouts complete and calls the callback, the next task is called.
        setTimeout(function () {
            callback(null, item + ' result');
        }, Math.random() * 100);
    }, this);
}, function (ctl, errs, results) { //Task 3
    console.log('Task 2 results: ' + results.join(', '));

    //Similar async operations as Task 2, but this time using counter.
    //The number 2 is for flowjs to know that once 2 callbacks
    //are called, proceed to next task.
    ctl.parallel(2, function (i, callback) {
        setTimeout(function () {
            callback(null, 'Async ' + (i + 1) + ' result');
        }, Math.random() * 100);
    }, this);

}, function (ctl, errs, results) { //Task 4
    console.log('Task 3 results: ' + results.join(', '));

    //No more tasks so no need to call ctl.next().
}
/*Any number of functions (tasks) can be added here.*/)();

Output:

Executing Task 1.
"Task 1 result"
Task 2 results: Async 1 result, Async 2 result
Task 3 results: Async 1 result, Async 2 result
How the example works:

Step 1: You are given a control object (ctl) that has API to continue to next task (ctl.next()) or do some parallel async operations and then continue to next task.

In Task 2 above, ctl.parallel is used, which gives a callback to be called once an async operation completes. The results/errors are passed through this callback so that the next task in the list gets them as it's input. Once all the async operations complete and all the callbacks are called, flow.js automatically executes the next task (which is "Task 3" in the example above). which get an array of results and an array of errors.

Step 2: In "Task 3", the results and errors which we got from the previous task is displayed.

Clean example

Now let's take a more real-world example to show you how clean the code becomes. Here is a flowchart containing some async operations a typical client-side application would need as part of it's "bootstrapping" logic:

Flowchart

You can easily put callbacks inside callbacks to write code for such a case, but when writing large pieces of code you'll end up with deep nested functions that will be hard to read later on. If you then try to move functions out into the parent scope using variables or named functions then the flow of execution of the code becomes harder to understand.

So now let's write the logic with flowjs.

window.CLIENTVERSION = 10;

flow(function (ctl) {
    //<Boostrapping code here>
    //<Check unsupported browsers here>

    //Fetch data version to check whether we need to migrate user or not.
    ajax('GET', '/version', ctl.next);
}, function (ctl, err, result) {
    if (err) { return; } //Handle error and return

    var response = result;
    if (response.dataVersion !== window.CLIENTVERSION) {
        //Initiate data migration
        ajax('GET', '/migrate', ctl.next);
    } else {
        ctl.next();
    }
}, function (ctl, err, result) {
    if (err) { return; }
    ajax('GET', '/data', ctl.next);
}, function (ctl, err, result) {
    if (err) { return; }
    //<Load application>
    //Done.
})();

function ajax(method, url, callback) {
    var xhr = new XMLHttpRequest();
    xhr.open(method, url, true);
    xhr.onload = function () {
        if (this.status >= 200 && this.status < 300) {
            callback(null, JSON.parse(this.resposeText));
        } else {
            callback(this.status);
        }
    };
    xhr.send();
}
Documentation

API Documentation

Advanced examples

Repeating tasks.
flow(function (ctl) {
    var repeat = (ctl.repeatCount < 2);
    //Do some parallel async operations and repeat this task two times.
    ctl.parallel({
        count: 2,
        repeat: repeat
    }, function (i, callback) {
        setTimeout(function () {
            callback(null, 'Async ' + (i + 1) + ' result');
        }, Math.random() * 100);
    }, this);
}, function (ctl, errs, results) {
    console.log(errs);
    console.log(results);
    console.log('Done.');
})();
Reusing tasks

I've come across a circumstance where I needed to split a task list into two so that one of them can be called again later.

var taskList1 = flow(...);

flow(function (ctl) {
    ctl.next(null, 'Results to pass to taskList1');
},
taskList1,
function (ctl, err, results) {
    console.log(results); //results from last callback of taskList1
})();

//You may execute taskList1() any number of times.
$('button').on('click', function () {
    taskList1();
});