JS Testing

Just grabbing some miscellaneous facts about the js tests.

tag: :js or type: :js in system and request tests make capybara use chrome headless, and wait for js.

https://github.com/forem/forem/blob/master/spec/support/initializers/capybara.rb#L29-L44

# adapted from <https://medium.com/doctolib-engineering/hunting-flaky-tests-2-waiting-for-ajax-bd76d79d9ee9>
def wait_for_javascript
  max_time = Capybara::Helpers.monotonic_time + Capybara.default_max_wait_time
  finished = false

  while Capybara::Helpers.monotonic_time < max_time
    finished = page.evaluate_script("typeof initializeBaseApp") != "undefined"

    break if finished

    sleep 0.1
  end

  raise "wait_for_javascript timeout" unless finished
end

interestingly, initializeBaseApp is the function that's awaited (or waited to be defined) - typeof is function - basically the file that's loaded defining this function (base.js.erb) also immediately calls the function - so the existence of the definition is evidence of its execution.

initializeBaseApp calls initializePage (or passes that to InstantClick, which I assume calls it on ready state?) - and we call initializeBaseApp at the end of that file:

// FUNCTIONAL CODE FOR PAGE

  function initializeBaseApp() {
    InstantClick.on('change', function() {
      initializePage();
    });
    InstantClick.init();
  }
  
  
  
var instantClick
  , InstantClick = instantClick = function(document, location, $userAgent) {
//  ... SNIP ...
  
    function on(eventType, callback) {
      $eventsCallbacks[eventType].push(callback)
    }
}


// Start BaseApp for Page
  initializeBaseApp()

In my "I don't understand javascript" confusion - I wondered why "call function initializePage by name" was something that needed a closure - but I believe, if I understand it correctly, the closure exists to provide access to a named function that's not exported/public/global - or might not be?

I personally, maybe it's schemelang leaking through, would assume this were expressed better as a simply function arg:

InstantClick.on('change', initializePage);