Don't use sleep or wait:

no_sleep

This breaks if we try to test via Capybara:

first(".modal-open").click 
first(".confirm").click
It will click the button but the next interaction will fail because the modal hasn’t finished loading.
We could add a sleep here in the test but this would slow the test down a lot and won’t guarantee that the modal is loaded.

Luckily, Capybara provides some helpers for this exact situation. 
Finders such as `first` or `all` return `nil` if there is no such element. 
`find` on the other hand will keep trying until the element shows up on the page or a maximum wait time has been exceeded (default 2 seconds).

# this will take a few seconds to open modal
find(".modal-open").click
# this will keep trying to find up to two seconds
find(".confirm").click

Similar to find, most of Capybara’s matchers support waiting. You should always
use the matchers and not try to call the query methods directly.

# This will _not_ retry
expect(page.has_css?(".active")).to eq false

# This _will_ retry if the element isn't initially on the page
expect(page).not_to have_active_class

Remember, feature specs test the application from a user’s perspective. 
As a user, I don’t care whether you use AJAX or not, that is an implementation detail.
Feature tests should assert on the UI only!

click_on "Save"

# This will automatically wait up to 2 seconds
# giving AJAX time to complete

expect(page).to have_css(".notice", text: "Document saved!)


Almost all AJAX interactions will change the UI in some manner for usability reasons. 
Assert on these changes and take advantage of Capybara’s auto-waiting matchers.