List a Page's Custom JavaScript Globals with a Bookmarklet

When you're debugging or reverse-engineering a site, it helps to know which global variables the page's own scripts created: configuration objects, framework instances, or library handles like jQuery or wp. This bookmarklet lists them in the browser console and saves them to window.$appGlobals for further inspection.

How It Works

The script creates a blank, hidden <iframe>. Its window object contains only the browser's built-in properties, so any property on the page's window that the iframe lacks was added by the page, its scripts, or an extension.

Comparing against a fresh iframe adapts to whichever browser and version you're using. Older versions of this bookmarklet hard-coded a list of built-in names, which went stale with every browser release.

The Script

Readable version:

(() => {
  const frame = document.createElement('iframe');
  frame.style.display = 'none';
  document.body.appendChild(frame);
  const builtins = new Set(Object.getOwnPropertyNames(frame.contentWindow));
  frame.remove();
  const globals = {};
  for (const key of Object.getOwnPropertyNames(window)) {
    if (!builtins.has(key)) {
      try { globals[key] = window[key]; } catch (e) { globals[key] = '(unreadable)'; }
    }
  }
  window.$appGlobals = globals;
  console.table(Object.keys(globals).sort().map((name) => ({ name, type: typeof globals[name] })));
  console.log(globals);
})();

Install It

1. Show the bookmarks bar (Ctrl+Shift+B, or Cmd+Shift+B on macOS).

2. Right-click the bar and choose Add page (Chrome, Edge) or Add Bookmark (Firefox).

3. Name it List Globals and paste this single line as the URL:

javascript:(() => { const frame = document.createElement('iframe'); frame.style.display = 'none'; document.body.appendChild(frame); const builtins = new Set(Object.getOwnPropertyNames(frame.contentWindow)); frame.remove(); const globals = {}; for (const key of Object.getOwnPropertyNames(window)) { if (!builtins.has(key)) { try { globals[key] = window[key]; } catch (e) { globals[key] = '(unreadable)'; } } } window.$appGlobals = globals; console.table(Object.keys(globals).sort().map((name) => ({ name, type: typeof globals[name] }))); console.log(globals); })();

Use It

Open the page, open the developer console (F12), and click the bookmark. The console shows a table of global names and their types. Then explore any of them:

$appGlobals.someConfigObject

Limitations

• Variables declared at the top level with let, const, or class don't become window properties, so they don't appear. Only var, function declarations, and explicit window.x = ... assignments do.

• Browser extensions that inject scripts into the page show up in the list too.

• Strict Content Security Policies can stop bookmarklets from running on some sites. If nothing happens, paste the readable version into the console instead.