test.ts

import puppeteer from 'puppeteer';

async function run() {
  const browser = await puppeteer.launch();
  const page = await browser.newPage();
  await page.goto('https://example.com');

  async function someHelper(element: puppeteer.ElementHandle<HTMLLIElement>): Promise<number> {
    return 2;
  }

  const stuff = await page.$('div')
  if (!stuff) {
    throw new Error('no stuff')
  }
  const foo = await stuff.evaluate((elem: Element) => {
   const x = elem.getAttribute('data-x') || '0'
   return window.parseInt(x);
  });
  console.log(foo)

  const listItem = await page.$('li') as puppeteer.ElementHandle<HTMLLIElement>;
  const result = await someHelper(listItem)
}

run()