Plugin System

This section outlines the various things needed to get you started with writing efficient Packem plugins. You must go through this section well enough to be able to build a Packem plugin. The examples are in TypeScript just so to keep an extra hand at the proper types of a variadic. If there are any issues or problems with this system, please file an issue or submit a PR.

Packem plugins are in-memory classes that are sequentially executed during build time. An instantiated plugin does absolutely nothing to the bundle cycle so therefore event hooks are needed to allow such manipulations.

The PackemPlugin class

The PackemPlugin class is an abstract class that stands as a container for newly created plugins. Plugins need to extend PackemPlugin to be able to use the event hooks.

As an example plugin, let’s create a plugin that logs ’🐶’ to the console everytime a JavaScript file is added to the module graph. Let’s start off with creating a class PuppyPlugin.

const { PackemPlugin } = require("packem");

// This does nothing! 😭
class PuppyPlugin extends PackemPlugin {}

module.exports = PuppyPlugin;

Using Event Hooks to manipulate the bundle cycle

To put more sense into our plugin, we’ll need an event that allows us to check if a JavaScript module is added to the module graph. The event onModuleBundle does exactly that. Event hooks are defined as class methods that might or might not take any arguments depending on the event type. In this case, onModuleBundle takes the module interface (more on this later) which allows us to read or write to the module graph.

const { PackemPlugin } = require("packem");

// This does nothing!
class PuppyPlugin extends PackemPlugin {
  onModuleBundle(mod) {
    if (mod.extension == "js") {
      console.log("You bet I have a cute 🐶!");

module.exports = PuppyPlugin;

Adding a plugin to packem.config.yml

One last step to avail your plugin to a project is adding it to the configuration file. In the packem.config.yml, add the following then run the command packem in the terminal:

root: ./src/index.js
output: ./dist
  packem-puppy-plugin: on

Depending on the number of JavaScript files in your project, you should’ve ended up with some logs in your terminal ("You bet I have a cute 🐶!"). Try adding more JavaScript files, rerun packem and see what happens.