Capybara gives you two different methods for executing Javascript:

page.evaluate_script("$('input').focus()")
page.execute_script("$('input').focus()")

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While you can use both, the first line (with evaluate_script) might freeze your browser window for 10 seconds.

The reason is that evaluate_script will always return a result. The return value will be converted back to Ruby objects, which in case of complex objects (e.g. a jQuery collection) is very expensive.

Because of this we recommend to only use evaluate_script when you require access to the result value.