const result = await frame.evaluate(() => {
  return Promise.resolve(8 * 7);
});
console.log(result); // prints "56"

You can pass a string instead of a function (although functions are recommended as they are easier to debug and use with TypeScript):

const aHandle = await page.evaluate('1 + 2');

To get the best TypeScript experience, you should pass in as the generic the type of pageFunction:

const aHandle = await page.evaluate<() => number>(() => 2);

const bodyHandle = await page.$('body');
const html = await page.evaluate(body => body.innerHTML, bodyHandle);
await bodyHandle.dispose();