Async function that fetches window variables into a Chrome content script (or another scope, in general).

fetchWindowVars.js

// Based on https://stackoverflow.com/a/55431653/13321349
function fetchWindowVarsPromise(varNames, wnd = undefined) {
    wnd = wnd || window;
    return new Promise((resolve, reject) => {
        // Generate random ID, to prevent other code from easily sending fake values
        const randomId = uuid4();

        // Register self-removing message handler first
        const _messageHandler = (event) => {
            console.log("DBG fetch vars:", event)
            if (event.source != wnd) return;
            const msg = event.data;
            if (msg.randomId == randomId) {
                event.preventDefault();
                wnd.removeEventListener("message", _messageHandler);
                resolve(msg.vars);
            }
        }
        wnd.addEventListener("message", _messageHandler);

        // Inject IIFE that sends message with variable values
        const script = wnd.document.createElement("script");
        script.type = "text/javascript";
        script.id = 'varMessageSender';
        script.textContent = `
            (function (randomId, varNames) {
                const msg = { randomId: randomId, vars: {} };
                for (const varName of varNames) {
                    msg.vars[varName] = window[varName];
                }
                window.postMessage(msg, '${wnd.origin}');
            })('${randomId}', ${JSON.stringify(varNames)});`;
        wnd.document.body.append(script);
    });  // end new Promise
}