Yii2 prevent form submit with vanilla js

i have a form in a modal, when i click on submit button i want to prevent the form submission using vanilla js.
But if i use preventDefault(); like this code

form.addEventListener('beforeSubmit', function(event) {
   event.preventDefault();
   //Ajax submit
})

i get 2 submit events in my form end the form is sent…
Instead if a use jquery

$("#formCalendar").on('beforeSubmit', function (event) {
      event.preventDefault();
      //Ajax submit
      return false;
})

It works…
Why with vanilla js doesn’t work?