Odd JavaScript Errors in Developer Tools

[jonathansampson]

Description

Users observing the developer tools may see strange JavaScript errors, potentially unrelated to any of their own code. For example:

VM91284:1 Uncaught TypeError: Cannot read property 'innerText' of null
    at <anonymous>:1:15

When the user clicks to examine the code which raised the exception, they see only the following:

document.body.innerText

Alternatively, they may see the following:

TypeError: Cannot read property 'requestContent' of undefined

Steps to Reproduce

Note: This issue is not very easy to reproduce; there definitely appears to be a race condition involved.

1. Navigate to https://sampson.codes/brave/bugs/innerText-of-null/
2. Open developer tools, and enable break on exception
3. Using CMD|Ctrl + R, refresh the page over and over until the page halts on an exception

Actual result:

Exceptions are raised by code other than the user's.

Expected result:

No exceptions are raised by any code other than the user's.

Reproduces how often:

Intermittent. Somewhat difficult to reproduce.

Brave version (brave://version info)

Brave	1.15.75 Chromium: 86.0.4240.99 (Official Build) (64-bit)
Revision	002668237e13d38aabd6d11c2d216dd22b736ff2-refs/branch-heads/4240@{#1229}

Version/Channel Information:

• Can you reproduce this issue with the current release? Yes
• Can you reproduce this issue with the beta channel? Yes
• Can you reproduce this issue with the nightly channel? Yes

Other Additional Information:

• Does the issue resolve itself when disabling Brave Shields? No
• Does the issue resolve itself when disabling Brave Rewards? Yes
• Is the issue reproducible on the latest version of Chrome? No

Miscellaneous Information:

The test-URL above is not always quick to produce the error. @jonathansampson has another URL, which can be shared privately via Slack. If you're working on this, and would like a more consistent endpoint to test, please ping him.

[jonathansampson]

This code appears all throughout the Chromium codebase, but it looks like this particular instance is coming from an attempt to leverage the dom_distiller for page classification.

Source: /browser/brave_ads/ads_tab_helper.cc

void AdsTabHelper::RunIsolatedJavaScript(
    content::RenderFrameHost* render_frame_host) {
  DCHECK(render_frame_host);

  dom_distiller::RunIsolatedJavaScript(
      render_frame_host, "new XMLSerializer().serializeToString(document)",
      base::BindOnce(&AdsTabHelper::OnJavaScriptHtmlResult,
                     weak_factory_.GetWeakPtr()));

  dom_distiller::RunIsolatedJavaScript(
      render_frame_host, "document.body.innerText",
      base::BindOnce(&AdsTabHelper::OnJavaScriptTextResult,
                     weak_factory_.GetWeakPtr()));
}

I suspect there is no body element at times, which causes the error. If this is the case, it may be appropriate to instead use optional chaining, like document?.body?.innerText.

[stephendonner]

Despite being unable to reproduce this using 1.15.75 (yet), I'm verifying using the inline testplan here, with build

Brave	1.27.33 Chromium: 91.0.4472.77 (Official Build) nightly (x86_64)
Revision	1cecd5c8a856bc2a5adda436e7b84d8d21b339b6-refs/branch-heads/4472@{#1246}
OS	macOS Version 11.4 (Build 20F71)

At the very least:

1. the changes are inclusive and minimal
2. I can't reproduce the problem with this build, regardless, so we should be good

example	example