page.evalute doesn't seem to work in version 0.13.0

Steps to reproduce

Tell us about your environment:

• Puppeteer version: 0.13.0
• Platform / OS version: macPS Sierra version 10.12.6

What steps will reproduce the problem?

  await page.goto(`${opts.appUrl}#${opts.localToken}`, {waitUntil: 'networkidle0'});
  
  const header = await page.evaluate(() => {
    const element = document.getElementsByClassName('header');
    console.log(element);
    return element;
  });
  console.log(header)

What is the expected result? I have an almost empty page with a header. If I run the following code the element is logged to the console in the browser in the page.evaluate function. However the next console.log is undefined. I expected it to be the same.

If I run this code in version 0.10.2 it works.

  await page.goto(`${opts.appUrl}#${opts.localToken}`, {waitUntil: 'networkidle'});
  
  const header = await page.evaluate(() => {
    const element = document.getElementsByClassName('header');
    console.log(element);
    return element;
  });
  console.log(header)

The only difference is that networkidle is replaced with networkidle0

[vsemozhetbyt]

There were some changes in result serialization before the 0.13. You can try this:

    const header = JSON.parse(await page.evaluate(() => {
      const element = document.getElementsByClassName('header');
      console.log(element);
      return JSON.stringify(element);
    }));

[ggorlen]

I think document.getElementsByClassName('header'); should be document.getElementsByClassName('header')[0]; or document.querySelector(".header")), else rename const element to const elements. See this SO answer which uses the same code.