Get notified when something (pattern, text) appears on a page. Great for pages showing logs !

waitForText.js

function sleep(ms) { return new Promise(resolve => setTimeout(resolve, ms)); }

function toArrayOfPatterns(searchPatterns) {
    // transform strings, patterns, or arrays of strings/patterns into arrays of patterns
    return (Array.isArray(searchPatterns) ? searchPatterns : [searchPatterns])
        .map((pattern) => typeof pattern == "string" ? new RegExp(pattern) : pattern);
}

async function isNotificationPermissionGranted() {
    // ask for permission to show notification
    return new Promise((resolve, _reject) => {
        const permissionDenied = () => console.log(
            "%cYou didn't grant notification permission for this page. Aborting.",
            "font-weight: bold; color: red");
        if (Notification.permission === 'granted') {
            resolve(true);
        } else if (Notification.permission === 'default') {
            console.log("Requesting notification permission.");
            console.log(
                "%cFROM FIREFOX 72 onwards%c\nIt is not possible to trigger the permission prompt without user interaction. " + 
                "A little icon should have appeared on the left of the URL bar though. Click on it to grant permission !\n" +
                "See https://blog.mozilla.org/futurereleases/2019/11/04/restricting-notification-permission-prompts-in-firefox/",
                "font-weight: bold; color: red", "");
            Notification.requestPermission().then(() => {
                Notification.permission === 'granted' ? resolve(true) : resolve(false);
            });
        } else {
            permissionDenied();
            resolve(false);
        }
    });
}

/**
 * Wait for some text/pattern to appear on the page, and show a notification.
 * The `searchPattern` can be a string, a pattern (`RegExp`), or an array of string/pattern.
 * If an array is passed, the notification is shown as soon as any string/pattern is found (logical OR).
 * 
 * @example 
 * waitForText([/hello.*worlds?/, "exact match"], document.getElementById("out"), {title: "My Title"})
 * 
 * @param {(string | RegExp | [(string|RegExp)])} searchPattern the pattern(s) to wait for for (logical OR)
 * @param {HTMLElement} element in which element to search for the string (default: the whole body)
 * @param {object} notificationParams notification param overrides (see https://developer.mozilla.org/en-US/docs/Web/API/notification)
 */
async function waitForText(searchPattern, element, notificationParams) {
    if (!await isNotificationPermissionGranted()) return;

    const elt = element ?? document.body;
    const patterns = toArrayOfPatterns(searchPattern);
    console.log(`Waiting for any of ${patterns}.`,
        "Type 'window.stopWaiting = true' on the developer console to stop the wait, or reload the page.");

    while (!window.stopWaiting) {
        console.log("..."); // show that something is working in the background
        let matchingPattern = patterns.find(pattern => pattern.test(elt.innerHTML));
        if (matchingPattern) {
            const match = matchingPattern.exec(elt.innerHTML)[0];
            console.log(`${match} found. Showing notification.`);
            // compute actual notification parameters
            let params = Object.assign({
                title: "Found!",
                body: `found "${match}" in ${document.title}`,
                requireInteraction: true // only dismiss on click (if supported)
            }, notificationParams);
            // show the notification
            let notification = new Notification(params.title, params);

            // switch automatically to the right window on notification click
            notification.onclick = function () {
                window.parent.parent.focus();
                notification.close();
            };
            return;
        }
        await sleep(5_000);
    }
    console.log("WaitForText aborted.");
    window.stopWaiting = false; // reset
}

// this can be used anytime to stop the current waiting loop
window.stopWaiting = false;

// attach functions to window so they are available from bookmarklets
window.waitForText = waitForText;

See the blog post https://dev.to/derlin/getting-notified-when-a-string-pattern-appears-on-a-webpage-318j for more information.