Concise, Powerful Asynchronous Flow Control in JavaScript

npm install ff
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ff: Concise, Powerful Asynchronous JavaScript Flow Control Build Status

ff simplifies the most common use cases for series, parallel, and promise utilities.


$ npm install ff

In the browser, simply add a script tag pointing to lib/ff.js in your HTML page.

Table of Contents

Quick Start

ff() accepts a list of functions to be run in sequential order, and returns an object that can be used to manage the flow of data between these functions. You may also pass in a context object as the first parameter, which FF will bind to all function calls.

var ff = require("ff");

var f = ff(this,
    function () {
        fs.readFile("1.txt", f.slot());
        fs.readFile("2.txt", f.slot());
    function (fileA, fileB) {
        this.sortFileContents(fileA, fileB, f.slot());
    function (result) {

FF is also Promises/A+ compliant. For more information about using FF promises, see below.

var f = ff(this,
    function () {
        fs.readFile("1.txt", f.slot());

    function onFulfilled(data) { }, 
    function onRejected(err) { }

A typical Express web handler looks like this. (Note that even if an exception gets thrown during one of these handlers, the .onError() handler will be called.

function (req, res, next) {
    var f = ff(
        function() {
            authenticateUser(req, f.slot());
        function (user) {
            f.pass(user); // pass the user along synchronously
        function (user, friends) {
            res.json({ user: user, friends: friends });
    ).onError(next); // call next() *only* on error

API Documentation

First, call ff and save its return value (as f, perhaps).

var f = ff([context], stepFunctions, ... )

The ff() function takes a context and any number of functions, which we call "steps". Each step is run one at a time. Use ff's return value (often called f) to manage the flow of data between functions.

Second, use the returned f object inside each step function.

Within your step functions, pass f.slot() as the callback parameter to any async function. This reserves a "slot" in the next step's function arguments. For instance:

fs.readFile("1.txt", f.slot()); // the file contents will be passed to the next function

Most often, that's all you'll need, but there are other ways to leverage FF to handle the flow of data.

f.pass(data); // pass data synchronously to the next function
fs.exists("1.txt", f.slotPlain()); // fs.exists doesn't pass (err, result), just (result)
emitter.once("close", f.wait()); // just wait for the "close" event, don't pass any data

All Methods on f:

f.slot() aliased as f()

Calling f.slot() reserves a slot in the next step's function arguments, and returns a callback that you should pass into an async function. The async function should be called with an error as in callback(err, result).

f.pass(arg1, arg2...) aliased as f(arg1, arg2...)

If you call f.pass(), the arguments will be passed into the next step. This can be useful when you need to pass along a value directly to the next function synchronously.


Sometimes you don't want to pass any arguments to the next function, but you just want to wait until an async call completes successfully. This behaves exactly like f.slot(), handling errors, but no data is passed to the next step.


This is like f.slot(), except that the resulting callback must not accept an error, as in callback(result). Node's fs.exists doesn't return an error, for instance, and so you must use f.slotPlain() for its callback instead. (If you had used f.slot(), it would have thought fs.exists had passed an error as the first argument.


See f.slotPlain(). Like f.wait(), this does not pass any arguments to the next step.


Like f.slot(), except that the resulting callback will pass n arguments to the next step instead of just one. For instance, calling var cb = f.slotMulti(2) followed by cb(err, rsp, body) would pass both rsp and body as two arguments to the next step.


This reserves exactly one slot in the next step, and returns a group object that has all of the above methods. Anything you slot or pass into the group gets passed into the next function's argument list as an array. (See the Groups example.)


This causes the chain of steps to end successfully (after you return from the current function). The result handlers (.onSuccess() and .onComplete()) will be called as soon as the current step returns. No other steps will be executed afterward.


This causes the chain of steps to end as though the given error had occurred (after you return from the current function). The result handlers (.onError() and .onComplete()) will be called as soon as the current step returns. No other steps will be executed afterward.


You can add additional steps after calling ff() using f.next(fn). Internally, we pass the arguments through this function initially.


Set a timeout; if the ff chain of steps do not finish after this many milliseconds, fail with a timeout Error. Works with both deferred and normal ff steps.

Finally, remember to handle the result! (.onComplete, .onError, .onSuccess)

After you've called ff() with your steps, you'll want to handle the final result that gets passed down the end of the function. We often do this like so:

var f = ff(
   // steps here...

That final callback will be passed arguments node-style: onComplete(err, results...). The number of arguments after err depends on how many slots you passed from the last function in the chain.

There are three ways you can handle the final result (and you can mix and match):

f.onComplete( function (err, results...) { } )

A .onComplete() result handler will always be called, whether or not an error occurred. An error object will be passed first (null if there was no error.)

f.onSuccess( function (results...) {} )

A .onSuccess() handler will only be called if no error occured. Additionally, an error object will not be passed. Only results.

f.onError( function (err) {} )

A .onError() result handler will only be called if an error occured. In this case, err will never be null. (If you're using Express, often we use .onError(next) to propagate whenever we didn't reach a call to res.send().)

Always remember to add one of these result handlers after your ff() call, so that errors propagate! You can add multiple result handlers and they will each be called in the order in which they were registered.

Error Handling

If any function throws an exception, or an error gets passed to one of the callbacks (as in callback(err, result)), the error will be propagated immediately to your result handlers (.onComplete() and .onError()). If a result handler throws an exception, that exception will bubble up into Node's unhandledException handler or the browser's developer console.

Advanced Usage

Groups (for processing arrays)

The f.group() method reserves exactly one slot in the next step and returns an object just like f. Anything you slot or pass into the group gets passed into the next function's argument list as an array. This is useful for processing arrays of items. Here's an example:

var allMyFiles = ["one.txt", "two.txt", "three.txt"];

var f = ff(
    function () {
        var group = f.group();
        allMyFiles.forEach(function (file) {
            fs.readFile(file, group());
    function (allFiles) {
        // allFiles is an array of 3 items (the contents of each file).

        // If any call had returned an err, this function would not be
        // called, and the error would have been passed down to `onComplete`.

Implementation Details

The following are equivalent:

var f = ff(this,
var f = ff(this);

Error handling is actually quite simple: If an error occurs in any step, it gets passed down to the onComplete or onError handler, skipping over any .next handlers.

Promise API (Deferreds)

ff can also be used as a promise library. If you are intersted in managing your own promises, you can use the defer helper.

var f = ff.defer();

// set callbacks:
    function onFulfilled(result, restul2) { },
    function onError(err) { }

// now trigger the result (or rejection)
f(result, result2); // or f.fail(err);

To trigger success or failure:

f(arg1, arg2...) // success
f.fail(err)      // failure

In addition to using then to attach completion handlers, you can also use the regular ff .onSuccess(), .onError(), and .onComplete() to do so.

And just like regular ff, you can pass functions into ff.defer(...):

var f = ff.defer(
    function(result, text) {
        // do something with result
    function () {
        // ...etc...

    function onFulfilled(results) { },
    function onError(err) { };

// now fire the result into the first step!
f(result, "something else");

If you want to know more about how ff promises work, see the Promises/A+ spec.

Quick Reference / Cheat Sheet

The API Documentation provides a much more thorough tutorial.

Control Flow API Summary

// Create a chain of steps with the `ff` function:
var f = ff(context, 
    function () {
        // Within each method, use the `f` object.
        // Most common uses:
        f(arg1, arg2); // pass multiple arguments synchronously
        fs.readFile("file1.txt", f());      // use f() for async callbacks
        fs.readFile("file2.txt", f.wait()); // just wait for the result
                                            // without putting it in args

        // To process arrays, use groups:
        var group = f.group();
        allFiles.forEach(function (item) {   // use any `f` function on arrays
            fs.readFile(item, group.slot()); // and the result gets stored as
        });                                  // an array in the next step

        // Less common uses for atypical functions
        fs.exists("file3.txt", f.slotPlain()); // fs.exists doesn't pass an error
        fs.exists("file4.txt", f.waitPlain()); // ditto, and I don't care if it fails
        var cb = f.slotMulti(2); // slot and pass two arguments to the next function
                                 // for example, cb(null, 1, 2);

        // Aborting the chain of steps early:
        f.succeed(result1, ...); // after this function, skip the other steps
        f.fail(err);             // after this function, fail with this error
        f.timeout(200);             // abort if it doesn't finish before 200 milliseconds
    function (arg1, arg2, file1, allFiles, file3Exists, multi1, multi2) {
        // Do something amazing here!
).onComplete(nextFn); // <-- usually you'll have someone else handle a (err, result...) callback

// Add a timeout (which would result in a failure with a timeout Error

// Don't forget all the result handler options (attach as many as you like!)
f.onComplete(function (err, args...) { }); // triggered on both success and error
f.onSuccess(function (args...) { }); // only on success
f.onError(function (err) { });       // only on error

Promise API Summary

// Create a deferred
var f = ff.defer();

// Add result handlers:
    function onFulfilled(arg1, ...) { },
    function onError(err) { };

// Trigger results: 
f(arg1, ...); // fulfill
f.fail(err);  // reject


Made by Marcus Cavanaugh and Michael Henretty.

This code was originally based on Tim Caswell's sketch of a reimagined Step library.

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