Access variables and functions defined in page context from an extension

javascript

script-injection

extension-development

page-context

byAlex Kataev·Oct 29, 2024

Access a page's DOM variables and functions from a chrome extension by:

Crafting a content script.
Injecting a <script> tag containing your custom code into the DOM.

Here's a quick look:

// In your content script
const script = document.createElement('script');
script.src = chrome.runtime.getURL('injected-script.js');
(document.head || document.documentElement).appendChild(script);

In injected-script.js:

// Knock knock! Accessing page-specific variables/functions
const pageFunctionResult = window.pageFunction();
console.log('Hey Page! We need to talk:', pageFunctionResult);

This links the isolated world of the extension to the page's environment.

Injecting scripts: A cautionary note

Injecting code into a page presents potential challenges:

Order: Sequence matters when loading scripts, especially when they're interdependent. You can control order by injecting scripts sequentially using the script.onload event.
Security: The page might redefine built-in prototypes or globals, potentially causing code failure or data exfiltration.

To handle these, use:

chrome.storage or chrome.runtime.sendMessage for secure messaging.
cloneInto to extend back to the page in Firefox, when dealing with objects that need accessing or modifying from the page context.

Advanced script interaction

To interact with page scripts such as YouTube videos, you can add the routes to the scripts in web_accessible_resources of manifest.json:

// Manifest.json
"web_accessible_resources": [
  "scripts/injected-script.js"
],

Here, you can also use Document.dispatchEvent and CustomEvent for page script interaction.

Debug: Mastering the art of finding needles in haystacks

Debugging script injection issues is just a console.log away. It's an art!

// What goes wrong stays wrong! (just joking. But seriously, let's catch those bugs)
console.log("Fire lit. Debugging started. Marshmallows, anyone?");

Script communication: A tale of two contexts

For achieving two-way communication between real DOM and content scripts, use onmessage and postMessage.

// Content script
window.postMessage({ type: "FROM_EXTENSION", text: "Hello from the other side." }, "*");

The real DOM can retrieve messages sent from within an extension using an onmessage handler in the same context:

// Real DOM
window.addEventListener("message", event => {
  if (event.source == window && event.data.type && event.data.type == "FROM_EXTENSION") {
    console.log("Message Received, over and out:", event.data.text);
  }
});

Continuous learning: The power-up of a developer

Better script management is achieved by a function repository where scripts from multiple sources are managed and injected. Also, with fresh JavaScript development practices being introduced, continuous learning armors you for effective extension development.

explain-codes / Javascript / Access variables and functions defined in page context from an extension

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