waitForFunction(pageFunction, arg[, options])

import { browser } from 'k6/experimental/browser';
import { check } from 'k6';

export const options = {
  scenarios: {
    browser: {
      executor: 'shared-iterations',
      options: {
        browser: {
            type: 'chromium',
        },
      },
    },
  },
}

export default async function () {
  const page = browser.newPage();
  
  try {
    page.evaluate(() => {
      setTimeout(() => {
        const el = document.createElement('h1');
        el.innerHTML = 'Hello';
        document.body.appendChild(el);
      }, 1000);
    });

    const ok = await page.waitForFunction("document.querySelector('h1')", {
      polling: 'mutation',
      timeout: 2000,
    });
    check(ok, { 'waitForFunction successfully resolved': ok.innerHTML() == 'Hello' });
  } finally {
    page.close();
  }
}