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WebExtension Webpack Example

A minimal example of how to use webpack to package npm modules so they can be used in a WebExtension. The example package used by this extension is left-pad, an essential package in almost any situation.

What it does

This example shows how to use a node module in a background script and popup. It defines two build targets in webpack.config.js, they each generate a file that includes all modules used the entry point and store it in the addon folder. The first one starts with background_scripts/background.js and stores it in addon/background_scripts/index.js. The other one does the same for popup/left-pad.js and stores it in addon/popup/index.js.

The extension includes a browser action with a popup, which provides an UI for running left-pad on a string with a chosen character. The operation can either be performed with the left-pad module included in the panel's script or in the background script.

What it could do

This could be infinitely extended - injecting global jQuery, adding babel, react/jsx, css modules, image processing, local modules and so on.

What it shows

  • How to use npm or custom modules in a WebExtension.

How to build it

  • npm install
  • npm run build

The WebExtension in the addon folder should now work.

What about Browserify?

Browserify works just as well as webpack for extensions. In the end it's a personal choice about your preferred tool.


As well as watching the folder with your manifest.json in it, you will also have to run webpack in watch mode. You can use the webpack-webext-plugin to simplify the workflow.

On Reviews

Files generated by webpack and friends are compiled files. You have to separately upload the source you generated your extension from to AMO for review of a listed extension. This will mean that your extension has to be reviewed by an admin reviewer, which will result in a longer wait time in queue.

To make the review easier, you can exclude third-party libraries from your output and directly ship the original distribution files for the libraries. This allows AMO to automatically recognize libraries and mark them as safe. This can be achieved with externals in the configuration for webpack.

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