Inject scripts into the active tab

# Overview

This tutorial builds an extension that simplifies the styling of the Chrome extension and Web store documentation pages so that they are easier to read.

In this guide, we’re going to explain how to do the following:

• Use the extension service worker as the event coordinator.
• Preserve user privacy through the activeTab permission.
• Run code when the user clicks the extension toolbar icon.
• Insert and remove a stylesheet using the Scripting API.
• Use a keyboard shortcut to execute code.

# Before you start

This guide assumes that you have basic web development experience. We recommend checking out Development Basics for an introduction to the extension development workflow.

# Build the extension

To start, create a new directory called focus-mode that will hold the extension's files. If you prefer, you can download the complete source code on GitHub.

# Step 1: Add the extension data and icons

Create a file called manifest.json and include the following code.

{
  "manifest_version": 3,
  "name": "Focus Mode",
  "description": "Enable focus mode on Chrome's official Extensions and Chrome Web Store documentation.",
  "version": "1.0",
  "icons": {
    "16": "images/icon-16.png",
    "32": "images/icon-32.png",
    "48": "images/icon-48.png",
    "128": "images/icon-128.png"
  }
}

To learn more about these manifest keys, check out the Reading time tutorial that explains the extension's metadata and icons in more detail.

Create an images folder then download the icons into it.

# Step 2: Initialize the extension

Extensions can monitor browser events in the background using the extension's service worker. Service workers are special JavaScript environments that are loaded to handle events and terminated when they're no longer needed.

Start by registering the service worker in the manifest.json file:

{
  ...
  "background": {
    "service_worker": "background.js"
  },
  ...
}

Create a file called background.js and add the following code:

chrome.runtime.onInstalled.addListener(() => {
  chrome.action.setBadgeText({
    text: "OFF",
  });
});

The first event our service worker will listen for is runtime.onInstalled(). This method allows the extension to set an initial state or complete some tasks on installation. Extensions can use the Storage API and IndexedDB to store the application state. In this case, though, since we're only handling two states, we will use the action's badge text itself to track whether the extension is 'ON' or 'OFF'.

# Step 3: Enable the extension action

The extension action controls the extension’s toolbar icon. So whenever the user clicks on the extension action, it will either run some code (like in this example) or display a popup. Add the following code to declare the extension action in the manifest.json file:

{
  ...
  "action": {
    "default_icon": {
      "16": "images/icon-16.png",
      "32": "images/icon-32.png",
      "48": "images/icon-48.png",
      "128": "images/icon-128.png"
    }
  },
  ...
}

# Use the activeTab permission to protect user privacy

The activeTab permission grants the extension temporary ability to execute code on the currently active tab. It also allows access to sensitive properties of the current tab.

This permission is enabled when the user invokes the extension. In this case, the user invokes the extension by clicking on the extension action.

The activeTab permission allows users to purposefully choose to run the extension on the focused tab; this way, it protects the user’s privacy. Another benefit is that it does not trigger a permission warning.

To use the activeTab permission, add it to the manifest's permission array:

{
  ...
  "permissions": ["activeTab"],
  ...
}

# Step 4: Track the state of the current tab

After the user clicks on the extension action, the extension will check if the URL matches a documentation page. Next, it will check the state of the current tab and set the next state. Add the following code to background.js:

const extensions = 'https://developer.chrome.com/docs/extensions'
const webstore = 'https://developer.chrome.com/docs/webstore'

chrome.action.onClicked.addListener(async (tab) => {
  if (tab.url.startsWith(extensions) || tab.url.startsWith(webstore)) {
    // Retrieve the action badge to check if the extension is 'ON' or 'OFF'
    const prevState = await chrome.action.getBadgeText({ tabId: tab.id });
    // Next state will always be the opposite
    const nextState = prevState === 'ON' ? 'OFF' : 'ON'

    // Set the action badge to the next state
    await chrome.action.setBadgeText({
      tabId: tab.id,
      text: nextState,
    });
...

# Step 5: Add or remove the stylesheet

Now it's time to change the layout of the page. Create a file named focus-mode.css and include the following code:

body > .scaffold > :is(top-nav, navigation-rail, side-nav, footer),
main > :not(:last-child),
main > :last-child > navigation-tree,
main .toc-container {
  display: none;
}

main > :last-child {
  margin-top: min(10vmax, 10rem);
  margin-bottom: min(10vmax, 10rem);
}

Let's insert or remove the stylesheet using the Scripting API. Start by declaring the "scripting" permission in the manifest:

{
  ...
  "permissions": ["activeTab", "scripting"],
  ...
}

Finally, in background.js add the following code to change the layout of the page:

  ...
    if (nextState === "ON") {
      // Insert the CSS file when the user turns the extension on
      await chrome.scripting.insertCSS({
        files: ["focus-mode.css"],
        target: { tabId: tab.id },
      });
    } else if (nextState === "OFF") {
      // Remove the CSS file when the user turns the extension off
      await chrome.scripting.removeCSS({
        files: ["focus-mode.css"],
        target: { tabId: tab.id },
      });
    }
  }
});

# Optional: Assign a keyboard shortcut

Just for fun, let's add a shortcut to make it easier to enable or disable focus mode. Add the "commands" key to the manifest.

{
  ...
  "commands": {
    "_execute_action": {
      "suggested_key": {
        "default": "Ctrl+B",
        "mac": "Command+B"
      }
    }
  }
}

The "_execute_action" key runs the same code as the action.onClicked() event, so no additional code is needed!

# Test that it works

Verify that the file structure of your project looks like the following:

# Load your extension locally

To load an unpacked extension in developer mode, follow the steps in Development Basics.

# Test the extension on a documentation page

First, open any of the following pages:

• Welcome to the Chrome Extension documentation
• Publish in the Chrome Web Store
• Scripting API

Then, click on the extension action. If you set up a keyboard shortcut, you can test it by pressing Ctrl + B or Cmd + B.

It should go from this:

To this:

# 🎯 Potential enhancements

Based on what you’ve learned today, try to accomplish any of the following:

• Improve the CSS stylesheet.
• Assign a different keyboard shortcut.
• Change the layout of your favorite blog or documentation site.

# Keep building!

Congratulations on finishing this tutorial 🎉. Continue leveling up your skills by completing other tutorials on this series:

# Continue exploring

We hope you enjoyed building this Chrome extension and are excited to continue your Chrome development learning journey. We recommend the following learning paths:

• The Chrome Extension Architecture backs up a bit and fills in a lot of detail about the Extensions architecture in general.
• The developer's guide has dozens of additional links to pieces of documentation relevant to advanced extension creation.
• Extensions have access to powerful APIs beyond what's available on the open web. The Chrome APIs documentation walks through each API.