WebUIBrowserTest supports preloading javascript libraries.  On the renderer side, the corresponding scripts end up stored and executed (around commit time) from ChromeRenderFrameObserver::webui_javascript_.  If such scripts report javascript errors, the errors are ignored.

Example where javascript errors happen, but tests pass:

    $ cat out/cros/args.gn 
    dcheck_always_on = true
    is_component_build = true
    is_debug = false
    target_os = "chromeos"
    use_goma = true

    $ ninja -C out/cros -j 1500 -l 25 chromevox_tests browser_tests
    ...

    $ DISPLAY=:20 out/cros/chromevox_tests --gtest_filter=*CvoxBrailleIntegrationUnitTest.WriteWithSpans*
    ...
    [115664:115664:1101/133609.258899:INFO:CONSOLE(153)] "Uncaught TypeError: Cannot read property 'getURL' of undefined", source: earcon_engine.js (153)
    [115664:115664:1101/133609.261122:INFO:CONSOLE(14)] "Uncaught TypeError: Cannot read property 'AutomationNode' of undefined", source: recovery_strategy.js (14)
    [115664:115664:1101/133609.263432:INFO:CONSOLE(16)] "Uncaught TypeError: Cannot read property 'AutomationNode' of undefined", source: automation_predicate.js (16)
    [115664:115664:1101/133609.264543:INFO:CONSOLE(21)] "Uncaught TypeError: Cannot read property 'AutomationNode' of undefined", source: automation_util.js (21)
    [115664:115664:1101/133609.266805:INFO:CONSOLE(65)] "Uncaught TypeError: Cannot read property 'AutomationNode' of undefined", source: cursors.js (65)
    [115664:115664:1101/133609.286202:INFO:CONSOLE(28)] "Uncaught TypeError: Cannot read property 'AutomationNode' of undefined", source: output.js (28)
    [115664:115664:1101/133609.421675:INFO:CONSOLE(1220)] "Running TestCase CvoxBrailleIntegrationUnitTest.WriteWithSpans", source: test_api.js (1220)
    ...
    [       OK ] CvoxBrailleIntegrationUnitTest.WriteWithSpans (2210 ms)
    [----------] 1 test from CvoxBrailleIntegrationUnitTest (2210 ms total)
    [----------] Global test environment tear-down
    [==========] 1 test from 1 test case ran. (2210 ms total)
    [  PASSED  ] 1 test.

    (the example is CrOS-only [sorry, that's the example what I run into] but based on reading the code it seems that the problem is OS-agnostic)


chrome/test/base/web_ui_browser_test.cc tries to detect javascript errors, by intercepting logs via base/logging.h, filtering out non-javascript logs and storing the errors in a global variable:

    base::LazyInstance<std::vector<std::string>>::DestructorAtExit error_messages_ =
        LAZY_INSTANCE_INITIALIZER;
    
    // Intercepts all log messages.
    bool LogHandler(int severity,
                    const char* file,
                    int line,
                    size_t message_start,
                    const std::string& str) {
      if (severity == logging::LOG_ERROR && file &&
          std::string("CONSOLE") == file) {
        error_messages_.Get().push_back(str);
      }

      return false;
    }

For some reason the errors from "preloaded" libraries do not impact test results.