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Preventing Default Behaviour

event.preventDefault() cancels the default action of a cancelable event: following a link, toggling a checkbox, submitting a form, inserting text. return false does the same only in an on... attribute or property.

Canceling default actionsJavaScript
link.addEventListener("click", (e) => e.preventDefault());          // stay on the page
form.addEventListener("submit", (e) => { e.preventDefault(); save(new FormData(form)); });
code.addEventListener("beforeinput", (e) => {
  if (e.data && /[^a-z]/.test(e.data)) e.preventDefault();         // lowercase letters only
});

beforeinput cannot be canceled for autocomplete or input-method text, so validate again on submit. A passive listener's preventDefault() is ignored.