Working of Browser Extensions

Introduction

Browser extensions may appear simple on the surface, often represented by a small icon or a popup window, but behind this simplicity lies a well-structured and powerful system. Understanding how browser extensions work is important for both users and developers, as it explains how extensions interact with web pages, access browser features, and operate securely. This chapter discusses the internal architecture, components, and mechanisms that enable browser extensions to function effectively.

Architecture of a Browser Extension

A browser extension is built using standard web technologies such as HTML, CSS, and JavaScript, and it follows a predefined architecture enforced by the browser. This architecture ensures that extensions remain modular, efficient, and secure.

The core components of a browser extension include:

• Manifest file
• Background scripts
• Content scripts
• User interface elements

Each component has a specific role and operates in a controlled environment.

The Manifest File

The manifest file is the most important part of a browser extension. It is a configuration file written in JSON format that tells the browser everything it needs to know about the extension.

The manifest file is the metadata file of a browser extension.

It contains metadata because it stores descriptive and configuration information about the extension, not the main logic.

Functions of the Manifest File:

• Defines the extension's name, version, and description
• Specifies permissions required by the extension
• Lists files such as scripts, icons, and UI pages
• Declares browser actions and events

Without a valid manifest file, a browser extension cannot run. The manifest acts as a bridge between the browser and the extension's internal components.

Example:

{
  "name": "Sample Extension",
  "version": "1.0",
  "description": "A simple browser extension",
  "manifest_version": 3,
  "permissions": ["activeTab"]
}

This file does not perform actions.

Instead, it describes:

• What the extension is
• What it can access
• How it should behave

Background Scripts

Background scripts handle tasks that need to run continuously or independently of web pages. They operate in the background and listen for events such as browser startup, tab changes, or user actions.

Responsibilities of Background Scripts:

• Managing long-running processes
• Handling alarms and timers
• Responding to browser events
• Communicating with other extension components

Background scripts do not interact directly with web page content. Instead, they coordinate the overall behavior of the extension.

Content Scripts

Content scripts are responsible for interacting with web pages. They are injected into web pages and can read or modify the page's content.

Capabilities of Content Scripts:

• Access and manipulate the Document Object Model (DOM)
• Read page information such as text or links
• Modify layout, styles, or content
• Detect user interactions on a webpage

However, content scripts run in a restricted environment and cannot directly access browser APIs, which helps maintain security.

User Interface Components

Browser extensions often include user interface elements that allow users to interact with them easily.

Common UI Elements:

• Toolbar icons
• Popup windows
• Option or settings pages
• Context menu items

These components are usually built using HTML and CSS and provides controls such as buttons, forms, or toggles for user interaction.

Permissions and Browser APIs

Browser extensions operate using a permission-based model. Users must approve permissions before an extension can access certain browser features.

Common Permissions Include:

• Access to tabs and windows
• Reading and modifying website data
• Using storage and notifications
• Accessing bookmarks or browsing history