This note was originally posted on my DevNotes

Browser extensions add features and functionalities to a browser. It uses web technologies such as HTML, CSS, and JS.

Manifest.json

The only file that a browser extension need is the manifest.json file.

Using manifest.json, you specify basic metadata about your extension such as the name and version, and can also specify aspects of your extension's functionality, such as background scripts, content scripts, and browser actions.
MDN

The manifest.json keys dependent on the extension you are developing.

You can find the available keys on manifest.json - MDN

Example:

{
  //Mandatory
  "manifest_version": 2,
  "name": "Name of your extension",
  "version": "1.0.0",

  // Recommended
  "default_locale": "en",
  "description": "Extension description",
  "author": "Your Name",
  "homepage_url": "https://github.com/vinceumo",
  "icons": {
    "16": "icons/icon16.png",
    "32": "icons/icon32.png",
    "48": "icons/icon48.png",
    "128": "icons/icon128.png"
  }
}

Adding and Debugging a browser extension locally

Firefox

Add extension

• Navigate to about:debugging
• Check Enable add-on debugging
• Click on Load Temporary Add-on
• Select the manifest.json file

Debugging extension

• Navigate to about:debugging
• Find your extension
• Click on Debug
• Developer toll will open

Chrome / Opera

Add extension

• Navigate to chrome://extensions/
• Click on Load unpacked
• Select the extension folder

Debugging extension

Background script errors can be shown when the package is loaded.

• Navigate to chrome://extensions/
• Find your extension
• Click on the errors button (Will show only if errors)
• Click on View in Developer Tools

To debug a popup extension:

• Open the popup
• Right-click in the popup
• Click on Inspect

Chrome - Debugging Extensions

Edge

Add extension

• Navigate to about:flags
• Check Enable extension developer features
• Click on the Settings and More icon (3 dots icon on the right)
• Select Extensions from this menu
• Click on Load extension
• Select the extension folder

Debugging extension

To debug a popup extension:

• Open the popup
• Right-click in the popup
• Click on Inspect element

Microsoft Edge - Debugging Extensions

Cross browser extension using a polyfill

mozilla/webextension-polyfill allows browsers to use Promise-based WebExtension APIs. Some browsers might style need some adjustments.

Note that opera use Chromium as well as Vivaldi and Brave, your extension will work in all these browsers if it works in Chrome.

Publish an extension

Chrome / Brave / Vivaldi

Brave and Vivaldi can download extensions from the Chrome web store.

• Create a zip file of your extension (Manifest file need to be at the root of this file)
• Connect/Create an account on the Google developer dashboard
  • If you submit an extension for the first time you will need to pay a $5 fee
• Upload the zip file

Firefox

• Create a zip file of your extension (Manifest file need to be at the root of this file)
• Connect/Create account on the Add-on developer hub
• Submit a new Add-on

Opera

• Create a zip file of your extension (Manifest file need to be at the root of this file)
• Connect/Create account on the opera add-ons platform
• Submit a new Add-on
• You will need to wait for moderation, could take up to a few weeks/months

Example of a popup extension

I developed recently the browser extension A11Y-Color-Blindness-Empathy-Test-browser-extensions.

This extension is a popup extension that injects an inline SVG for filtering colours, some style and changes the class of the html tag of the current tab open, depending on the picked radio button.

The extension will follow the following APIs and structure:

• manifest.json - permissions
• manifest.json - Browser_action
• WebExtensions API - tabs
• Content scripts

WebExtension
|   manifest.json // Give permissions to the active Tab and reference the popup `choose_test.html`
|
+---content_scripts
|       runTest.js // Script interacting with the active tab
|
+---icons
|       icon128.png
|       icon16.png
|       icon32.png
|       icon48.png
|
+---polyfills
|       browser-polyfill.js // Polyfill for chrome and opera
|
\---popup
        choose_test.css // Style of the popup
        choose_test.html // Markup of the popup
        choose_test.js // Script of the popup, click handler

Resources

• MDN - Browser Extensions
• Chrome - Browser Extensions
• Microsoft Edge extension API roadmap
• W3C - Browser extensions
• webextensions-examples - Github
• How to make a cross-browser extension using JavaScript and browser APIs - Medium