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Welcome to modifying web requests! This lesson introduces declarativeNetRequest, a powerful API for controlling network requests in your browser extension.

You'll learn how to define rules to block unwanted content like ads, tracking scripts, or specific resources, making web browsing cleaner and faster.

Blocking network requests isn't just about privacy; it's also about performance and user experience.

• Remove Ads: Block requests to ad servers.
• Stop Trackers: Prevent tracking scripts from loading.
• Improve Performance: Less data downloaded means faster page loads.
• Enhance Security: Block malicious domains or content.

Traditional ad blockers often use imperative logic: JavaScript code that runs on every page to detect and block elements.

declarativeNetRequest is declarative. You define rules, and the browser handles the blocking efficiently, often without waking up your extension's JavaScript. This is faster and more resource-friendly!

To use declarativeNetRequest, your extension needs to declare the permission in its manifest.json file.

This tells the browser your extension intends to modify network requests.

A blocking rule is an object with an id, an action, and a condition. The browser checks these conditions against every network request.

• id: A unique number for the rule.
• action: What to do (e.g., 'block', 'redirect').
• condition: When to apply the action (e.g., 'urlFilter', 'resourceTypes').

For this lesson, we'll focus on the "block" action. When a request matches a rule with this action, the browser simply prevents that request from completing.

The action part of your rule will look like this:

The urlFilter property in a rule's condition allows you to specify a pattern for URLs to match. It's like a mini-regex.

• "*://*.example.com/*" matches any subdomain of example.com.
• "*://ads.google.com/*" specifically targets Google ads.
• "*track*" matches URLs containing 'track'.

You can also specify which types of resources to block using resourceTypes. This helps you target specific content like images, scripts, or fonts.

Common types include "script", "image", "stylesheet", "media", and "font".

Rules can be added dynamically by your extension using chrome.declarativeNetRequest.updateDynamicRules(). This is often done in your background script.

This example adds a rule to block requests containing "adserver" in their URL, specifically for scripts and images.

Consider the following blocking rule:

Which of these requests would be blocked?

Great job! You've learned the basics of blocking network requests with declarativeNetRequest.

• It's a powerful, performant, and privacy-friendly API.
• Rules consist of an id, action (e.g., "block"), and condition.
• urlFilter and resourceTypes are key for defining conditions.
• Dynamic rules can be added and managed using updateDynamicRules().

Next, you'll explore modifying headers!