Evaluate returns undefined but chromium correctly has value inside evaluate

[chrisabrams]

Steps to reproduce

Tell us about your environment:

• Puppeteer version: 1.3.0
• Platform / OS version: OS X Sierra 10.12.6
• URLs (if applicable):
• Node.js version: 8.10.0

What steps will reproduce the problem?

You can run this gist here: https://gist.github.com/chrisabrams/a2a098d1b2d697f349f4e1ad1be98602

What is the expected result?
The expected result is that evaluate will return the list of nodes

What happens instead?
Instead, evaluate returns undefined

[vsemozhetbyt]

If I get this right, DevTools protocol cannot serialize an array of nodes and resolves it to undefined. If you make it an array of strings, this works:

return nodes.map(({ innerText }) => innerText)

[vsemozhetbyt]

See in the doc:

If the function passed to the page.evaluate returns a non-Serializable value, then page.evaluate resolves to undefined.

[yanivefraim]

How about throwing an error when evaluate returns a non-Serializable value? cc @aslushnikov

[chrisabrams]

Got it @vsemozhetbyt. I would concur on adding a error when the value being returned is not serializable.

[aslushnikov]

@yanivefraim @chrisabrams we were there at one moment, but had to change to undefined because there are many evaluations that happen for the sake of evaluation rather then return value.

await page.evaluate(() => window.saveValue = document.body);

We consider returning a special value in this case instead to emphasize the difference.

[chrisabrams]

@aslushnikov I see what you're saying. In my case, I need to know the saveValue outside of the evaluation. I can make serialization work though.

[aslushnikov]

In my case, I need to know the saveValue outside of the evaluation. I can make serialization work though.

@chrisabrams why don't you use the page.evaluateHandle then?