Plugin API

Plugin API has a very powerful mechanism of manipulating the output.

Let's take a look a plugin's interface first

interface Plugin {
    test?: RegExp;
    opts?: any;
    init?: { (context: WorkFlowContext) };

    transform: { (file: File, ast?: any) };
    transformGroup?(file: File): any;
    onTypescriptTransform?: { (file: File) };

    dependencies?: string[];

    preBundle?(context: WorkFlowContext);
    bundleStart?(context: WorkFlowContext);
    bundleEnd?(context: WorkFlowContext);
    postBundle?(context: WorkFlowContext);

    // available, but not implemented yet
    preBuild?(context: WorkFlowContext);
    postBuild?(context: WorkFlowContext);


test [RegExp]

Defining test will filter files into your plugin. For example \.js$ If specified you plugin's transform will get triggered upon transformation. It's optional. Experiment with regex101 to see it will match.


dependencies a list of npm dependencies your plugin might require. If provided, then the dependencies are loaded on the client before the plugin is invoked. For example this case


Happens when a plugin is initialized. It is common practice to reset your plugin state in this method.


transform if your plugin has a test property, fusebox will trigger transform method sending [file][src-file] as a first argument.



Happens on bundle start. A good place to inject your custom code here. For example here


All files are bundled. But it has not been finalized and written to a file.


Triggered after adding shims.


Triggered after the bundle source has been finalized, but before it is written to file. UglifyPlugin uses this trigger.

see the source code that triggers these plugin methods

Alternative content

If for some reason we want to preserve file contents for a later reuse and override the output, we can use file.alternativeContent which affects directly bundling process over here

It can be use for the concat technique for example

If an array of plugins is passed, those plugins will be chained

    plugins: [
        [fsbx.LESSPlugin(), fsbx.CSSPlugin()],




  • file.contents: can rewrite the output of a particular chunk, it is a simple string
  • information about the file, such as and
  • file.analysis.dependencies: can clear/flush dependencies of a file by setting it to an empty array.
  • read the File source for more


To use the AST, you need to know if the AST has been loaded already. You can do this by checking whether file.analysis.ast is not undefined.

If it has not been loaded, it can be loaded by doing:


If the babel plugin has been used, the AST will be loaded using babel babylon.

You can load any ast parser you'd like by

if (!file.analysis.ast) {
  const result = YourAST(file.contents)

read the FileAnalysis source for more

Transforming typescript

You can tranform typescript code before it actually gets to transpiling

const MySuperTranformation = {
    onTypescriptTransform: (file) => {
        file.contents += "\n console.log('I am here')";
    plugins: [MySuperTranformation],

Concat files

It is possible to concat files into one using the plugin API. There is ConcatPlugin which serves as an example for the subject.

In order to understand how it works imagine a plugin chain:

[/\.txt$/, fsbx.ConcatPlugin({ ext: ".txt", name: "textBundle.txt" })],

We have 2 files, a.txt and b.txt which are captured by the plugin API, and each of them is redirected to the ConcatPlugin's transform, which looks like this:

public transform(file: File) {
    // Loading the contents of file a.txt or b.txt

    let context = file.context;

    // create a file group in the context with name which is set in the plugin configuration
    // let's say "txtBundle.txt"
    let fileGroup = context.getFileGroup(this.bundleName);
    if (!fileGroup) {
        fileGroup = context.createFileGroup(this.bundleName);
    // Adding current file (say a.txt) as a subFile

    // making sure the current file refers to an object at runtime that calls our bundle
    file.alternativeContent = `module.exports = require("./${this.bundleName}")`;

When we register a new file group context.createFileGroup("txtBundle.txt") FuseBox creates a fake or a virtual file which is added to the dependency tree. This file has a special mode, called groupMode.

We need to alter the output as well using alternative content. Original contents will be ignored by the Source bundler.

After FuseBox has bundled all files related to your current project, it checks for groups over here, iterates and executes plugins. Then each plugin is tested accordingly (now our file name is called txtBundle.txt with .txt extension) and executes transformGroup of a plugin if set.

You should understand that txtBundle.txt behaves like any other file, with one exception - it does not call transform but tranformGroup instead.

public transformGroup(group: File) {
    let contents = [];
    group.subFiles.forEach(file => {
    let text = contents.join(this.delimiter);
    group.contents = `module.exports = ${JSON.stringify(text)}`;

Now our bundle has a virtual file which looks like this:

___scope___.file("textBundle.txt", function(exports, require, module, __filename, __dirname){
    module.exports = "hello\nworld"

Things to experiment with

  • try changing the contents of a file
  • try logging the contents of the file in the transformation file
  • try logging the ast once it has been loading

Plugin API source code