How to Develop a Chrome Extension: Expert Guide

What Is a Chrome Extension?

In today's digital era, web browsers play a significant role in our lives, serving as gateways to an infinite realm of information and possibilities. The leading player in the browser market these days is, with no doubt, Google Chrome. One of the reasons behind Chrome's popularity is its flexibility and customizability, allowing users to personalize their browsing experience through the use of extensions.

Chrome extensions are small software programs that enhance browser's functionality to suit the specific needs of users. These programs may offer a wide range of features and functionalities like blocking ads on websites you visit, managing your passwords and many more addons that create custom experiences.

This Chrome extensions tutorial will lead you through the fundamentals of creating these handy browser enhancements. Find out more about how to build a Google Chrome extension and create a Google Chrome Extension based on our guide!

Reasons to Build Chrome Extension

Browser extensions are microprograms that extend the functionality of the browser, they’re also capable of being integrated into cloud software through the browser and expanding its functionality.

The main reason to build extensions is clear from the name itself - to expand the functionality of the browser. A plugin can be used to integrate its elements into any cloud service for the purpose of integration, additional capabilities for SaaS, creating a microservice, or simply as an additional tool.

You can use browser extensions to personalize your browser, block ads on websites, translate the content of a website into another language, or add any website to a third-party bookmarking service such as Evernote or others.

Reasons to build a Chrome extension are clear from the advantages they bring to the table:

• Quick access, ease and clarity of use;
• Cross-platform nature: the ability to work on any platform where there is a browser;
• The ability to integrate any functionality and insert it into third-party products;
• The ability to combine your systems and cloud services into a comprehensive corporate landscape of systems.

Thus, with their simplicity and functionality, they deserve special attention.

​​Getting Started With Chrome Extension

To begin with, creating Google Chrome extensions requires a basic understanding of web technologies such as HTML, CSS, and JavaScript. These are the building blocks that enable you to interact with the browser and modify its behavior.

Another crucial part in building a Google Chrome extension is following best practices and adhering to Chrome's guidelines to ensure compatibility and security. Google provides extensive documentation and resources for software engineers, including code samples, tutorials, and a robust development community.

​​Chrome Extension’s Basic Structure

Behind the scenes, every extension is built upon a well-defined structure that serves as the foundation for its functionality.

Chrome extensions consist of various components, including manifest files, background scripts, content scripts, and user interface elements. Let’s observe some details of a proper extension structure.

A Golden Standard of Chrome Extensions

The most important rule software engineers should always remember when working with Chrome Extensions architecture is that all business and functionality logic should be on the extension side. Other endpoints, such as content script and external script are just UI for your background script functionality.

Background script

The core of any Chrome extension coding lies here. This script is a kind of service worker that listens for getting messages from content scripts and outworld web apps. Background script emits functions that do business functionality “under the hood” of extension. All functionality should be implemented here as event listener handlers.

Content script

The content script is necessary if we want to render something on the third party websites or inject some code there. It is enabled when the page is fully loaded. For example, it can be used to gather information from websites and then operate with it within the extension.

Structure example

To illustrate the process further, let's consider a sample Chrome extension that displays inspirational quotes on every new tab. To create Google Chrome extension, you could start by setting up the manifest file with the necessary permissions and identifying the background script to handle the quote generation and storage. Then, you can develop a content script to inject the quote into each new tab. Finally, you can design a visually appealing user interface, such as a popup window, to allow users to customize the quotes or access additional features.

Steps of Creation a Chrome Extension

How to write Chrome Extensions? Let’s consider the important steps in this Chrome Extension guide:

1. Define Extension Purpose and Features:

First step of Chrome Extension development guide is to clearly outline the purpose and functionality of your extension. Understand what problem it solves or what value it adds to users.

2. Set Up Your Development Environment:

Ensure you have a code editor (e.g., Visual Studio Code) and a working knowledge of HTML, CSS, and JavaScript.

3. Create Manifest File:

To make a Chrome extension, you need a manifest file (manifest.json). This file outlines essential details about your extension, such as its name, version, permissions, and scripts.

4. Create HTML, CSS, and JavaScript Files:

Develop the necessary files for your extension. For example, if your extension has a popup, create popup.html, popup.css, and popup.js.

5. Test Your Extension Locally:

Load your extension locally by navigating to chrome://extensions/, enabling "Developer mode," and clicking "Load unpacked." Select the directory containing your extension files.

6. Add Functionality:

Implement the core functionality of your extension using JavaScript. Handle events, manipulate the DOM, and interact with Chrome APIs as needed.

7. UI/UX Design:

Ensure your extension has a user-friendly interface and adheres to Chrome's design guidelines. Consider using icons, colors, and layouts that align with the overall Chrome experience.

8. Testing and Debugging:

Thoroughly test your extension in various scenarios to identify and fix bugs. Utilize Chrome DevTools for debugging.

Chrome Extensions vs Web Applications

As you can see in our Chrome Extensions development guide, this is quite a unique software kind because its web app has some significant advantages in comparison with regular web applications:

• It is on “all the time” when the client’s browser is on (unless the user disabled the extension).
• It has access to security and privacy API such as website permissions or a kind of “firewall” API. Thus Chrome extension is the ultimate solution to the user privacy problem.
• It has access to hardware monitoring API, which makes Chrome extension a possible solution for hardware care-based applications.

Learn how much it costs to develop an app!

What Will Our Chrome Extension Look Like?

To develop a Chrome extension, you need to follow a simple scheme. For example, in our Chrome extension tutorial we will show how to build a simple extension that changes the background color of the webpage.

Manifest File

The manifest.json file is the core of your Chrome extension. It defines the name, version, permissions, and the files of Chrome extension.

{
  "manifest_version": 3,
  "name": "Background Color Changer",
  "version": "1.0",
  "description": "A simple Chrome extension to change the background color of the current page.",
  "permissions": ["activeTab"],
  "action": {
    "default_popup": "popup.html",
    "default_icon": {
      "16": "icons/icon16.png",
      "48": "icons/icon48.png",
      "128": "icons/icon128.png"
    }
  }
}

Popup HTML

The popup HTML file defines the user interface that appears when the user clicks the extension's icon.

<!DOCTYPE html>
<html>
  <head>
    <title>Background Color Changer</title>
    <style>
      body { width: 200px; }
      button { width: 100%; padding: 10px; margin-top: 10px; }
    </style>
  </head>
  <body>
    <button id="changeColor">Change Background Color</button>
    <script src="popup.js"></script>
  </body>
</html>

Popup JavaScript

The popup JavaScript file contains the logic for changing the background color when the user clicks the button.

document.getElementById('changeColor').addEventListener('click', () => {
  chrome.tabs.query({ active: true, currentWindow: true }, (tabs) => {
    chrome.scripting.executeScript({
      target: { tabId: tabs[0].id },
      function: () => {
        document.body.style.backgroundColor = 'lightblue';
      }
    });
  });
});

This basic Chrome extension example shows the general structure of a Chrome extension and its interaction patterns. Based on this template, you can create more complex solutions.

Key Features of Chrome Extensions

Let’s take a closer look at the most relevant features of Chrome Extensions.

Security and privacy: permissions management feature

It is worth mentioning that Google named an API related to permission as contentSettings API. So, googling it would be quite challenging.

So what exactly does this API do? This API allows us to manage and inspect current permissions states for websites where the user manages permission.  You can use this API for setting permissions for particular websites and for getting permissions related to these websites.

It opens the door for us to develop custom business logic that will monitor and do some actions that could prevent users from sharing important permissions, for example, by blocking some dangerous permissions for non-trusted websites. So, when following Chrome Extensions developer guide, don't forget to think about security.

Security and privacy: custom “firewall”

The core API of this feature is declarativeNetRequest, provided by Google. This API allows us to block showing and executing on even just some domains/websites via primitive URL filters but blocking showing and executing particular resources of certain domains/websites. It’s quite powerful in terms of creating a true privacy-friendly and advanced secure ecosystem in the browser for the user.

It is worth mentioning, that declarativeNetRequest support 2 different approaches for setting rules for blocking:

1. Dynamic rules. When rules should be fetched from some server and parsed/mapped by us in chrome extension and after that parsed rules will be pushed into declarativeNetRequest active rulesets property
2. Static rules. When rules should be set into the .json files and built into the project.

The first part of this unique system is that we will convert all rulesets into regex expressions for the URL filter and for all resource filters. This move allows us to perform matching in a handy manner and dodge many problems between url filtering syntax of declarativeNetRequest rules and string comparison in JS.

And finally, the most important and complicated part of this system is analyzing all resources for all domains and searching matching between resources of particular websites and resources that should be blocked. So we would gather all resources that exist in the particular websites via injected scripts that actually run into this website with window.performance.getEntries().

But here comes a new problem - gathering resources should be performed into injected scripts, but statistics gathering and calculations should be performed on the Chrome extension side.

We can’t sync actions on injected scripts and Chrome extensions. So we should use quite a unique solution - gather resources on the injected script side and send it to the background of the Chrome extension via Chrome message passing API.

Hardware care: complex solution

The hardware care feature is relevant when it comes to the usage of portable devices. This feature should massively improve the user experience when it works in the browser, especially for laptops with battery problems.

Let’s briefly observe a feature that provides automatic smart discarding (suspension) for browser tabs which will do economic energy and computational resources of the computer while it is surfing the Internet (almost all the time). Firstly, we should talk about architecture levels in this feature (sorted from low to top):

1. Low-level hardware service, which gets raw info about CPU and memory loading  directly from hardware via the system. Especially complicated services which calculate cpu loading percents because in raw form, this information actually has the following format:

‍

So we used advanced calculation formulas to handle this raw data and provide relevant information to the next level.

2. Middle-level service which provides discard handler functionality. Actually, the discard  handler functionality is a system that creates an async timer that, once it is finished, checks if a particular tab should be discarded (suspended). If yes, then discard this, and if no - runs a new instance of itself. Also, discard handlers invoke gathering statistics right before discarding and right after discarding. This information is necessary because we will need to calculate the benefits of discarding every tab.

3. High-level services that calculate statistics. This calculation could be invoked by the middle level - discard handler and do many data verifying, consistency checking, etc., processing and applying necessary structure to this information. And finally, save it to local storage.

Besides, we should implement advanced statistics for this feature which should include working with different timestamps and time periods, averaging information etc.

The Drawbacks of Building a Chrome Extension

The main problem with the Chrome extensions is the cybersecurity. Many, if not most, users do not take the time to read Chrome extension messages and automatically click Add to Chrome to be able to use the plugin. All this situation gives cybercriminals the opportunity to distribute adware or malware that masquerade as harmless extensions.

As for adware extensions, the right to modify the displayed content allows the display of advertisements on the sites you visit. Also, the extension can analyze users’ searches and other data.

When it comes to malicious extensions, access to the content of all visited sites allows cybercriminals to steal users’ credit card information, cookies, and other sensitive information. That is why, when developing a Chrome extension, it’s important to clearly inform users about the purpose and work mode of your Chrome extension. This will build trust between users and your platform. For more detailed guidance, the Google Chrome extension developer guide provides extensive resources and best practices for how to create a Chrome extension.

Conclusion

Chrome extension is an extremely useful and powerful web development tool that offers a new development perspective. It gives us access to unique and powerful API, which opens a bunch of possibilities for us, especially in the security and privacy domain.

When it comes to Chrome extension development, partnering with a skilled and experienced software engineering company can be a game-changer. Experienced software engineers would bring a broad expertise to the table, ensuring that your Chrome extension is crafted with precision and tailored to your specific requirements.

At Axon, we excel in understanding the intricacies of Chrome extension’s architecture along with following best practices and Chrome’s guidelines, ensuring that your extension meets the necessary standards for compatibility and security.

Don’t hesitate to contact us to get started and receive a free consultation on how to develop Chrome extensions!

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