Ghostly Loader

A wepback loader which allows developers to write alternate versions of your code (AKA ghosts), which are inserted into your codebase during bundling.

Developers can then control which version of their code executes at run time.

Try using ghostly for:

• A/B/C... testing
• Running a different set of code given a certain context (e.g. locale, localStorage)

Installation and Use With Webpack

The loader is built to work in tandem with the ghostly-controller, which is a library for helping you orchestrate when your ghosts should appear:

npm i --save-dev ghostly-loader
npm i --save ghostly-controller
or 
yarn add -D ghostly-loader
yarn add ghostly-controller

ghostly-controller is not actually required to use ghostly, more on this below.

In webpack.config.js

module.exports = {
  // ...
  module: {
    rules: [
      // ...
      { test: /\.js$/, use: 'ghostly-loader' },
    ],
  }

}

Usage

As of writing, ghosts can be created only with function declarations.

If in your source code you have a function that returns a greeting:

src/index.js

require('ghostly-controller')();

function myGreeting() {
    return "Hello!"
}

Then, in a separate file, you can write a ghost which will return a different greeting. Just make sure that the substitute code is in a file named with the pattern [target_file_name].ghost.js:

src/index.ghost.js

function myGreeting$spanish() {
    return "Hola!"
}

ghostly-controller places a function called determineGhost on the global scope. This function will be called with the name of the ghost and any configuration that the ghost function was created with. By default, ghostly-controller uses localStorage to help manage your ghosts; run webpack, then open localStorage in your dev tools to see the result. Try overwriting the data to control which version is run.

You can choose not to use ghostly-controller and define your own implementation in its place.

determineGhost will return either 'main' or the tag of the ghost that should be run in its place.

When 'main' is returned, the original or primary function will be used.

When the function and tag name are returned, the corresponding ghost will be used.

Multiple Ghosts

A single function may have many ghosts, they simply have to start with the name of the original function:

src/index.ghost.js

function myGreeting$spanish() {
    return "Hola!"
}

function myGreeting$german() {
    return "Hallo"
}

root

global.determineGhost = (functionName, config) => {
  assert(functionName === 'myGreeting')
  const ghostNames = Object.keys(config) // ['myGreeting$spanish', 'myGreeting$german']

  const randomIndex = Math.floor(Math.random() * ghostNames.length)

  return ghostNames[randomIndex]
}

Inline Configuration

Ghosts can be configured to run probalistically using inline comments.

Any text placed in a comment directly above your ghost will be sent to determineGhost when the function is called:

some-file.ghost.js

// 0.5
function myFunction$example() {
    return "I should be used half (0.5x) the time!"
}

In determineGhost:

global.determineGhost = (functionName, config) => {
  assert(functionName === 'myFunction')
  assert(config.myFunction$example === 0.5)

  if (functionName === 'myFunction') {
    return (Math.random() > config.myFunction$example) ? 'myFunction$example' : 'main'
  }
  
  return 'main';
}

General Syntax

function myFunction$optionalTagName () {
    // this code will run in place of the primary code.
}