I would like to have a file in my React application where I can store constants and functions for the whole app.

I declared it like so:

import dayjs from 'dayjs';

const constantsAndFunctions = {
    dateFormat:'DD.MM.YYYY',
    dateFormatWrittenOut:'D. MMMM YYYY',
    dateFormatSQL:'YYYY-MM-DD',
    dateTimeFormatSQL:'YYYY-MM-DD HH:mm',
    timeFormatSQL:'HH:mm',

    formatDateToWrittenOut : (date) => {
        //return dayjs(date).format(self.dateFormatWrittenOut); WHY U NO WORK?!
        return dayjs(date).format('D. MMMM YYYY');
    },
};

In my function formatDateToWrittenOut I wanted to call the key dateFormat, but it seems to be empty. No error messages in the console.

What am I missing out? Or is this idea not good in the first place?