Features of the Chrome API You Should Know

The Chrome extension APIs provide web developers with a rich set of tools for extending and enhancing the functionality of the Chrome browser. With these APIs, you can interact with web pages, add UI elements to Chrome, and even modify the behavior of the browser itself.

In this article, we‘ll explore some of the most important and useful features of the Chrome API that every extension developer should be familiar with. Whether you‘re building your first extension or looking to take your skills to the next level, understanding how to leverage these powerful APIs is key.

Overview of Chrome Extension APIs

At a high level, extensions are made up of HTML, CSS, JavaScript, and other web assets that are packaged up and loaded into Chrome. Extensions have access to a variety of special-purpose APIs that allow them to interact with the browser and perform functions not available to regular web pages.

Some of the core goals extensions can accomplish with the APIs include:

• Adding UI elements and functionality to web pages
• Observing and intercepting web requests
• Interacting with and manipulating browser tabs
• Storing data locally in the extension
• Adding items to context menus, the omnibox, and more

There are dozens of APIs available to extensions, but some of the most fundamental and commonly used ones include the browserAction, contextMenus, storage, tabs, and webRequest APIs. Let‘s dive into each of these in more detail.

browserAction

The browserAction API allows extensions to add icons to the main Chrome toolbar, to the right of the address bar. These icons can be clicked to trigger a popup, execute an action, or toggle a state in the extension.

There are a few key things to know about browser actions:

• You can specify different icons in different sizes for various pixel densities (16px, 24px, 32px)
• Each extension can only have one browser action
• The popup can contain any HTML content you want and have its own separate scripts

Here‘s an example of how to set up a basic browser action in the manifest.json file:

{
  "name": "My Extension",
  ...
  "browser_action": {
    "default_icon": {
      "16": "icon16.png",
      "24": "icon24.png",
      "32": "icon32.png"
    },
    "default_title": "Click me!",
    "default_popup": "popup.html"
  }
  ...
}

Then in your popup.html file you can add any content and functionality you need.

Browser actions are one of the main entry points to interacting with your extension, so it‘s important to design meaningful and helpful popups to guide the user.

contextMenus

The contextMenus API, as you might expect, lets you add new items to the right-click menu in Chrome. Context menu items are a great way to expose functionality in a convenient place for users.

You can create multiple context menu items organized in a hierarchy and specify what contexts they should appear in (like right-clicking a page vs an image vs a link). You can also add icons and separators to organize your menu items.

Here‘s a simple example of adding a context menu item that appears when right-clicking a page:

chrome.contextMenus.create({
  title: "My Menu Item", 
  contexts:["page"], 
  onclick: function(info, tab) {
    console.log("Item clicked!", info, tab);
  }
});

This will add an item titled "My Menu Item" to the context menu when right-clicking on a page. The onclick callback will be called when it is clicked, receiving information about the click and the current tab.

Context menus are a powerful way to integrate your extension naturally into the user‘s browsing flow.

storage

The storage API provides extensions with a way to persist data locally in Chrome using an API similar to localStorage. Data stored here is retained even when Chrome is closed and reopened.

There are a few key things to understand about the storage API:

• All stored data is scoped to your extension only
• The storage is asynchronous, so you need to provide callbacks
• There is both local storage (per-machine) and sync storage (synced with Chrome sign-in)
• The storage is not encrypted, so don‘t store sensitive user data

Here‘s an example of saving and reading some data with the storage API:

// Save data
chrome.storage.local.set({key: value}, function() {
  console.log(‘Value is set to ‘ + value);
});
// Read data 
chrome.storage.local.get([‘key‘], function(result) {
console.log(‘Value currently is ‘ + result.key);
});

Using the storage API, you can easily persist extension state or user preferences across browser sessions.

tabs

As you would expect, the tabs API provides a variety of functionality for interacting with and manipulating browser tabs. With it you can do things like:

• Get information about existing tabs
• Create, update, move, reload, and remove tabs
• Capture the visible tab as an image
• Detect changes to tabs in real-time

Here‘s a simple example of capturing the currently active tab:

chrome.tabs.captureVisibleTab(null, {}, function(dataUrl) {
  console.log(dataUrl);
});

This will capture the visible area of the currently active tab as a data URL that you could then display or manipulate in your extension.

The tabs API makes it possible to build very powerful tab management and productivity tools as extensions.

webRequest

The webRequest API allows extensions to observe and analyze traffic, and even intercept, block, or modify requests in-flight.

With the webRequest API you can:

• See the type, URL, method, headers, etc of requests
• Get information about request initiators and targets
• Modify request and response headers
• Redirect or block requests
• Provide authentication credentials

The webRequest API is very powerful and can enable some advanced use cases like ad blocking, traffic debugging, modifying site behavior, and more. Here‘s an example of blocking requests to a particular domain:

chrome.webRequest.onBeforeRequest.addListener(
  function(details) {
    return {cancel: details.url.indexOf("://www.evil.com/") != -1};
  },
  {urls: [""]}, 
  ["blocking"]
);

This uses the onBeforeRequest event to observe all requests and cancel those that are going to www.evil.com.

Take care when using the webRequest API as it can be quite invasive and has privacy, security, and performance implications if not used thoughtfully.

Permissions and Other APIs

As you might have gathered, extensions have to request permission to use most of the Chrome APIs. Permissions are specified in the manifest file. Some permissions can be requested at runtime rather than install time, and the user will be prompted to allow or deny access. It‘s a good practice to only request the permissions you actually need.

In addition to the APIs already covered, there are many more available to extensions, such as:

• bookmarks – Create, organize, and manipulate bookmarks
• browserSettings – Modify global browser settings
• commands – Add custom keyboard shortcuts
• cookies – Get and set cookies
• downloads – Initiate, monitor, and search for downloads
• history – Interact with the browser history
• notifications – Create rich notifications
• omnibox – Add customized address bar suggestions
• pageAction – Dynamically display icons in the toolbar based on page state

There are too many to cover in detail here, but the Chrome developer docs provide excellent references for learning more about all the available APIs.

Conclusion

In this post, we explored some of the key features and APIs available for developing Chrome extensions. Chrome provides extensions with a very powerful set of capabilities for modifying and enhancing the browsing experience.

To review, some of the most important Chrome extension APIs to know are:

• browserAction for adding UI to the toolbar
• contextMenus for adding right-click menu items
• storage for client-side data persistence
• tabs for working with and manipulating Chrome‘s tabs
• webRequest for observing and modifying web traffic

Of course, there are many more APIs to explore and combine in interesting ways. Hopefully this article has given you a good sense of what‘s possible and a starting point for digging deeper into the Chrome extension platform. The official docs are the best place to go for complete references and guides.

Happy extension building!

References

– Chrome extension development overview: https://developer.chrome.com/extensions/overview
– Chrome extension API reference: https://developer.chrome.com/extensions/api_index
– Chrome extension samples: https://github.com/GoogleChrome/chrome-extensions-samples