JSFiddle - React, Tailwind, and code Playground

HTML

<button id="download_calendar_btn">
  Download Calendar
</button>
<script>
var saveAs=saveAs||function(e){"use strict";if(typeof e==="undefined"||typeof navigator!=="undefined"&&/MSIE [1-9]\./.test(navigator.userAgent)){return}var t=e.document,n=function(){return e.URL||e.webkitURL||e},r=t.createElementNS("http://www.w3.org/1999/xhtml","a"),o="download"in r,a=function(e){var t=new MouseEvent("click");e.dispatchEvent(t)},i=/constructor/i.test(e.HTMLElement)||e.safari,f=/CriOS\/[\d]+/.test(navigator.userAgent),u=function(t){(e.setImmediate||e.setTimeout)(function(){throw t},0)},s="application/octet-stream",d=1e3*40,c=function(e){var t=function(){if(typeof e==="string"){n().revokeObjectURL(e)}else{e.remove()}};setTimeout(t,d)},l=function(e,t,n){t=[].concat(t);var r=t.length;while(r--){var o=e["on"+t[r]];if(typeof o==="function"){try{o.call(e,n||e)}catch(a){u(a)}}}},p=function(e){if(/^\s*(?:text\/\S*|application\/xml|\S*\/\S*\+xml)\s*;.*charset\s*=\s*utf-8/i.test(e.type)){return new Blob([String.fromCharCode(65279),e],{type:e.type})}return e},v=function(t,u,d){if(!d){t=p(t)}var v=this,w=t.type,m=w===s,y,h=function(){l(v,"writestart progress write writeend".split(" "))},S=function(){if((f||m&&i)&&e.FileReader){var r=new FileReader;r.onloadend=function(){var t=f?r.result:r.result.replace(/^data:[^;]*;/,"data:attachment/file;");var n=e.open(t,"_blank");if(!n)e.location.href=t;t=undefined;v.readyState=v.DONE;h()};r.readAsDataURL(t);v.readyState=v.INIT;return}if(!y){y=n().createObjectURL(t)}if(m){e.location.href=y}else{var o=e.open(y,"_blank");if(!o){e.location.href=y}}v.readyState=v.DONE;h();c(y)};v.readyState=v.INIT;if(o){y=n().createObjectURL(t);setTimeout(function(){r.href=y;r.download=u;a(r);h();c(y);v.readyState=v.DONE});return}S()},w=v.prototype,m=function(e,t,n){return new v(e,t||e.name||"download",n)};if(typeof navigator!=="undefined"&&navigator.msSaveOrOpenBlob){return function(e,t,n){t=t||e.name||"download";if(!n){e=p(e)}return...

JavaScript

const newEvent = {
    BEGIN: 'VCALENDAR',
    PRODID: 'www.website.com',
    UID: 'info@website.com',
    CATEGORIES: 'APPOINTMENT',
    DTSTART: '20140321T153010Z',
    DTEND: '',
    SUMMARY: 'My great event',
    DESCRIPTION: 'Great event in your town',
    END: 'VCALENDAR',
}
 
let formattedDate = formatDate(newEvent.DTSTART)

cal = ics()
cal.addEvent(newEvent.SUMMARY, newEvent.DESCRIPTION, newEvent.PRODID, formattedDate, formattedDate)

function formatDate(dateString) {
  return dateString.replace(/(\d{4})(\d{2})(\d{2})T(\d{2})(\d{2})(\d{2})/,"$1-$2-$3T$4:$5:$6")
}
  
  download_calendar_btn.onclick = function() {
    cal.download("my_calendar")
  }