"event: function(frm)" vs "event(frm)" event handler declaration style in client-side script

When declaring an event handler in a client-side script either:

frappe.ui.form.on("DocType", {
    event: function(frm) {


frappe.ui.form.on("DocType", {
    event(frm) {

style appear to work interchangeably.

Are they always interchangeable? Or are there any occasions or specific reasons to favour one style of declaration over the other?

Is the frm argument always the same as cur_frm within the handler function?

Appreciate your thoughts.