Capybara: Test that JavaScript runs without errors - ruby-on-rails

Question

I would like to write a request spec that verifies the loading and execution of javascript on a given page without any errors.
I know I can add something to the DOM at the end of my JavaScript file and assert the presence of this content, but that feels like a hack and forces me to pollute my code for testing reasons. I'd prefer to do something along the lines of.
visit some_path
page.should succesfully_run_javascript

Answer 1

You can achieve this with the following piece of code in your tests.
page.driver.console_messages.each { |error| puts error }
expect(page.driver.console_messages.length).to eq(0)
The first line prints out an errors, handy for seeing what's going on. The second line causes the test to fail.

Answer 2

One way that's potentially not very polluting is to collect any errors by adding something like this to head (keep in mind this will override any onerror handler you may already have):
<script>
window.errors = [];
window.onerror = function(error, url, line) {
window.errors.push(error);
};
</script>
You could then do something like:
page_errors = page.evaluate_script('window.errors')
and assert on the array being empty. You could output the errors otherwise...
Note: It's important to add the scriptlet to head (potentially as the first scriptlet/script tag) as it needs to be one the first executed scripts.

Answer 3

A problem with using page.driver.console_messages is that it doesn't clear between tests. So if you want to only assert that there are no errors on a particular test the spec might fail due to a previous test.
You can scope these errors to a particular spec by saving the last console.log timestamp.
Helper method:
def assert_no_js_errors
last_timestamp = page.driver.browser.manage.logs.get(:browser)
.map(&:timestamp)
.last || 0
yield
errors = page.driver.browser.manage.logs.get(:browser)
.reject { |e| e.timestamp > last_timestamp }
.reject { |e| e.level == 'WARNING' }
assert errors.length.zero?, "Expected no js errors, but these errors where found: #{errors.join(', ')}"
end
And then use it like:
def test_somthing_without_js_errors
assert_no_js_errors do
# TODO: Write test
end
end