Accessing `window`/DOM/HTML of the webpage from the extension

Problem: extension pages (popup, options, background page in MV2, etc.) are separate from the web page and they have their own DOM, document, window, and a chrome-extension:// URL.

• Note that service worker doesn’t have any DOM/document/window at all.
• To inspect each context of the extension use its own devtools.

Solution: use a content script to access the web page or interact with its contents.

• Content scripts execute in the web page, not in the extension.
• Content scripts are isolated by default, see how to run code in page context (aka MAIN world).
• Don’t load your content scripts in the extension page.

Method 1. Declarative

manifest.json:

"content_scripts": [{
  "matches": ["*://*.example.com/*"],
  "js": ["contentScript.js"]
}],

It will run once when the page loads. After that happens, use messaging .

Warning! It can’t send DOM elements, Map, Set, ArrayBuffer, classes, functions, and so on. It can only send JSON-compatible simple objects and types so you’ll need to manually extract the required data and pass it as a simple array or object.

Method 2. Programmatic

• ManifestV3:

  Use chrome.scripting.executeScript in the extension script (like the popup) to inject a content script/function into a tab on demand.

  The result of this method is the last expression in the content script so it can be used to extract data. Data must be JSON-compatible, see the warning above.

  Required permissions in manifest.json:

  • "scripting" – mandatory;
  • "activeTab" – ideal scenario, suitable for a response to a user action (usually a click on the extension icon in the toolbar). Doesn’t show any permission warning when installing the extension.

  If ideal scenario is impossible add the allowed sites to host_permissions in manifest.json:

  • "*://*.example.com/" plus any other sites you want.

  • "<all_urls>" or "*://*/" these will put your extension in a super slow review queue in the Chrome Web Store because of broad host permissions.

• ManifestV2 differences to the above:

  • Use chrome.tabs.executeScript.
  • Specify sites in permissions.