How Can I Access Global Variables in Gmail Messages Using a Chrome Extension?

Accessing Global Variables in Gmail Messages with a Chrome Extension

Introduction

Creating Chrome extensions can empower you with the ability to modify web page behavior. One such need is to retrieve global variables from the page, like the GLOBALS variable in Gmail messages.

The Issue

Unfortunately, content scripts run in an isolated environment, preventing direct access to the page's window properties like GLOBALS. jQuery's .load() function fails to retrieve it, resulting in ReferenceErrors. This occurs despite being able to access GLOBALS through the developer tools' console.

Solution: Cross-Context Communication

To bridge this communication gap, you can employ one of two methods:

1. Script Injection:

Inject a new script element into the page context. This script can retrieve the desired data and pass it back to the content script.

2. Event Listeners:

Use event listeners to pass data between the page and the content script. The content script can listen for custom events fired by the page script with the desired information.

Implementation Example Using Script Injection

Content Script (run_at: "document_end"):

<code class="js">var s = document.createElement('script');
s.src = chrome.extension.getURL('script.js');
(document.head || document.documentElement).appendChild(s);
s.onload = function() {
    s.remove();
};</code>

Script.js (Injected Script):

<code class="js">setTimeout(function() {
    document.dispatchEvent(new CustomEvent('RW759_connectExtension', {
        detail: GLOBALS
    }));
}, 0);</code>

Considerations

• Ensure "script.js" is added to the web_accessible_resources section of the manifest file.
• Content scripts should handle the majority of the logic, as they have access to additional features unavailable in the injected scripts.

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