Eat Your Greens - A Broccoli.js tutorial

Welcome to the world of Broccoli.js. So, I'm sure you're wondering, what is Broccoli.js?

Well, per the Broccoli.js website

The asset pipeline for ambitious applications

Cool, wait, what the hell does that mean?

This is a tutorial series of the workshop I gave at EmberConf 2018, follow along to learn more about Broccoli.js and how you can use it to build your javascript project.

"Ambitious applications"? That sounds scary, I just wanna make a simple JS app, with ES6 transpilation, SCSS pre-processing, asset concatenation, live reload, uglification, vendor npm module inclusion, development server, and, oh, wait, perhaps it's ambitious.

Oh, yeah, and I want all that to be fast.

Historically, we had grunt, and decided we didn't like configuration files. Then we had gulp, because we wanted to write code to compile our code, then it got slow. Then we had webpack that does bundling, minification, source maps, code splitting, but, it's kinda hard to configure. So Broccoli.js provides a simple Javascript API to do simple and complex file transformations, and do it fast.


Each step of the tutorial has an accompanied branch, so you can checkout the branch if you get lost and you should be able to build the project and see the output. Remember to run yarn when checking out new branches. All the documentation is in the docs folder, split into sections.

An overview presentation of Broccoli is available here: Presentation



What is a build tool responsible for? Well, it's job is to take input files (your javascript, css, html, etc) and process them, to output them into some form of distributable version. Typically this will involve things like javascript transformations to allow you to use newer syntax that will work in a browser, to use things like Sass for your CSS, etc.

So, how is Broccoli any different? Broccoli is a simple build management tool, that allows you to configure your build pipeline in javascript (like you're already used to writing), and handles setting up the filesystem state for each transformation that's going to happen.

Broccoli is structured around a concept of nodes and plugins, that allows the build pipeline to be structured somewhat like a "tree", with one or more inputs at the top, and a final output directory at the bottom, containing the result of the build.

Nodes reference one or more input directories, and an output directory. The output directory is automatically created by Broccoli itself.

Plugins are third party provided npm packages, that take one or more input nodes (directories), perform some kind of transformation, and emit files to an output path. This output path can then become an input path to the next node in the chain, and so on.

Nodes can then be manipulated via various broccoli plugins, to do things like copy files to the destination directory, pre-process files and convert them from one format to another (e.g. .scss to .css), merge directories together (think rsync'ing one directory into another), concatenate files of a certain type into one (bundling), uglifying, and so on.

How do nodes work?

Nodes are what Broccoli uses to represent snapshots of directories and the transformations between states.

Note that the term tree in Broccoli is often used to refer to a node and was previously used to refer to them (so don't get confused if you see the term tree used in other documentation).

Nodes themselves don't actually contain file contents, and manipulations to the nodes don't happen when they are created. They're essentially abstract representations of state, they can be combined, split, filtered for specific file types, etc.

Broccoli handles creating the node graph, that is, connecting the output paths of one or more nodes, to the input paths of another that will be consumed by the next plugin.

Nodes are always one of two types, source nodes and transform nodes.

source nodes represent a single input directory, and are implicitly created when you use a string as an input to a plugin. Typically source nodes are "watched" directories, and changes to any files within them will trigger a rebuild of their node "tree". You can also create unwatched directories for things like vendor files that don't change often.

transform nodes represent one or more input nodes, and are typically created by the output of plugins. Transform nodes delegate to a callback object (a plugin) during build time.

How do plugins work?

State transformations are performed by plugins. Plugins receive one or more input nodes, and write their output to the provided outputPath that Broccoli creates. Broccoli then takes the contents of outputPath and provides that as the inputPaths for the node to the next plugin.

Plugins take at least one input path, perform some processing on the files within those input paths, and write their result to the provided outputPath and Broccoli handles wiring all this up for you. This way each plugin is only concerned with the transformation that it needs to do.

Plugins can also cache their output, based usually on the hashes of their input paths, or any other factor. This way if the inputs to a plugin have not changed, then the plugin can supply a previously built output.

Plugins are very simple JS classes that accept one or more input nodes, and implement a build() method that is called when broccoli performs a (re)build.


You will certainly encounter the term "tree" when looking for Broccoli documentation. This was an often-used term that has now been replaced with "node", as node reflects the data structure better than a "tree". With this in mind, I prefer to use the term "tree" to refer to an entire build pipeline, or a collection of nodes and plugins.

An example Broccoli.js tree is represented in the following image:

Broccoli Pipeline

From the output node (the one at the bottom), a tree structure can be derived.


Broccoli build pipelines are defined using a Brocfile.js file in the root of the project. This js file defines the source nodes, passes them through various plugins creating transform nodes, and finally returns a single node that represents the final output of the build. Broccoli will then handle wiring up all of the nodes inputs and outputs into a graph (from the end node up to the start nodes), creating temporary directories as it goes, linking files between plugins using symlinks to make them super fast, run the build and invoke the build() method on each plugin, and finally resolve all the symlinks and write the files from the final node into the destination build directory.

Confused? Here's an example:

const mergeTrees = require("broccoli-merge-trees"); // broccoli merge-trees plugin
module.exports = mergeTrees(["dir1", "dir2"]);

This is a very simple Brocfile.js that merely merges the contents of dir1 and dir2 into the output directory. The node graph would be represented as follows:

source node
=====> transform node
source node
/dir1 => source node 1
/dir2 => source node 2
'dir1', => source node, implicitly created when using a string as an input
'dir2' => source node, implicitly created when using a string as an input
module.exports = transformation node with input nodes dir1 and dir2

Thus module.exports contains a node that references the two input nodes, and an output path that will contain the contents of dir1 and dir2 when the build command is run. The two input nodes reference two source directories, dir1 and dir2.

It sounds like a lot, but it's actually quite simple, so let's get started with a simple Broccoli app.


One last thing. Broccoli comes with a built in dev server, that provides an HTTP server to host your assets in development, and perform rebuilds when source directories (nodes) change.

To run the server, do:

broccoli serve

This should start a local HTTP server on http://localhost:4200

Cool, now you should have the basics down, let's go ahead and get a dummy project setup.

Step 1 - setup