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Feri - Custom Build Task

Feri comes with a lot of great build tasks by default but sometimes you need something a bit more specialized. That is where custom build tasks come in. Custom build tasks must return a Promise but before we go into specifics, lets review how Feri builds files behind the scenes.

Build tasks are defined in Each extension has an array of one or more tasks. The tasks can be strings or functions. Strings signify that the build task exists in the build module. Functions signify a custom build task.

For example, here is the entry for CoffeeScript: = ['coffeeScript', 'js']

In the above array, the two strings mean run any .coffee files through build.coffeeScript and then build.js. The first task transpiles CoffeeScript to JavaScript. The second task minifies the resulting JavaScript.

If instead, our source to destination tasks for CoffeeScript looked like...

function magicSauce() {
    // do magical things
} = ['coffeeScript', magicSauce]

We would surmise that any .coffee files would first run through build.coffeeScript and then a custom build task called magicSauce.

Reusable Object Building

Before any task defined in is run, a command and control function called build.processOneBuild creates an object that will be passed between build tasks for each file. Assuming the source file we are building is called hello.txt, the object would look like the following.

obj = {
    'source': '/source/hello.txt',
    'dest': '',
    'data': '',
    'build': false

The property source is the source path and file name. This field will always be filled out.

The property dest is the destination file path and name. This is typically figured out by the first build task.

The property data is used to pass strings between build functions that work in memory. Functions that write to disk will not use this field.

The property build will be set to true if a file needs to be built.

Obviously, we want our custom build task to receive a reusable object. For extra points we will also make sure it returns the reusable object so it can be chained with other build tasks.

Define a Custom Build Task

Let's imagine we want a workflow that will replace instances of a string {name} with Susan in text files.

Hello {name}! Lovely weather we are having today.

Using code from build.html as our template, we can simplify our custom build task down to the following:

function nameReplace(obj) {
    return feri.functions.objBuildInMemory(obj).then(function(obj) {

        if ( {
   ='{name}', 'Susan')
            return obj
        } else {
            // no further chained promises should be called
            throw 'done'


// don't forget to assign your custom build task to a file extension = [nameReplace]

First, we create a function called nameReplace that expects an object.

Next, we leverage a really neat Promise called functions.objBuildInMemory that does quite a few things for us.

  • It figures out that the destination path should be /dest/hello.txt and writes that to obj.dest.
  • Assuming the destination file does not exist or if the source file is newer than the destination, it sets to true.
  • Assuming is true, it reads the source file content into

Scenario I: Build is True

When our promise returns, we have an object like the following:

obj = {
    'source': '/source/hello.txt',
    'dest': '/dest/hello.txt',
    'data': 'Hello {name}! Lovely weather we are having today.',
    'build': true

Wow, our job is actually pretty easy now.

With set to true we run a simple replace on and then return the entire obj. If another build task receives our obj it will see the following:

obj = {
    'source': '/source/hello.txt',
    'dest': '/dest/hello.txt',
    'data': 'Hello Susan! Lovely weather we are having today.',
    'build': true

You may be thinking, wait a second... we didn't actually write a file and you're right. Every chain of build tasks has a special finisher task called build.finalize that takes care of writing our files to disk for us. So convenient!

If we instead choose to write to disk ourselves, it would be a good idea to clear before passing it along to any subsequent tasks like build.finalize. That way data isn't written twice.

Scenario II: Build is False

Let's say our promise returns an object like the following:

obj = {
    'source': '/source/hello.txt',
    'dest': '/dest/hello.txt',
    'data': ''
    'build': false

There is no reason to return the object for any further build tasks. Knowing there is nothing to do, we can throw 'done' to break out of our promise chain in a nice way. With less work, Feri runs faster!

More Complex Tasks

When building more complex tasks, your best friends will be functions.objBuildInMemory, functions.objBuildOnDisk, and functions.objBuildWithIncludes. Each of these is used in various bundled build tasks so feel free to use any of built-in tasks as a starter for your own awesome thing!


MIT © Daniel Gagan