Format defaults to mm/dd/yyyy unless stated in all constructors because it's not possible to set up default options

[szotsaki]

In Bootstrap datepicker you could set up defaults before initialising an instance in the following way:

$.fn.datepicker.defaults.todayHighlight = true;
$.fn.datepicker.defaults.language = "en-GB";
[…]

This option is missing from the Vanillajs version, unfortunately. If you don't want to one by one specify the format (like the non-US ISO format) in all of your Datepicker constructors, there's no other option.

• It's not possible to use setOptions() because it's called after construction is done, and the date will be parsed in a bad way initially. setOptions() will reformat that bad date to the nicer ISO date.
• Overriding the en locale doesn't work

[mymth]

I think it’s not really a bootstrap-datepicker's "feature" and you hacked because it was visible and modifiable due to JavaScript language's specs.

Anyway, how about doing like this?

class MyDatepicker extends Datepicker {
  constructor(element, options = {}, rangepicker = undefined) {
    super(element, Object.assign({
      todayHighlight: true,
      language: 'en-GB',
    }, options), rangepicker);
  }
}

// ...

const elem = document.querySelector('input.date');
const datepicker = new MyDatepicker(elem);

https://codepen.io/mymth/pen/WNaxrJV