JSFiddle - React, Tailwind, and code Playground

JavaScript

$.ajax({
	  url: "/ajax-reserveren/",
	  dataType: "json",
	  success: function(calendarEvents){ 
		  	$("#reserveren").datepicker({
	            minDate : '+0d',
	            numberOfMonths : 2,
	            hideIfNoPrevNext : true,
	            dateFormat : 'dd-mm-yy',
	            beforeShowDay: function(date) { 
	               var tooltip = null;
	               var avail = true;
	               var classes = null;
	               for (i = 0; i < calendarEvents.length; i++) {
						if (date.getMonth() == calendarEvents[i][0] - 1
						&& date.getDate() == calendarEvents[i][1]
						&& date.getFullYear() == calendarEvents[i][2]) {
                          
							if (calendarEvents[i][3] == 1) {
								classes = "ui-state-paid";
								tooltip = "";
								avail = false;
							} else if (calendarEvents[i][3] == 2) {
								classes = "ui-state-reserveerd";
								avail = true;
								tooltip = "Deze reservering is al reeds geboekt maar nog niet betaald";
							}
									 		
							}
                  }
	               return [avail,classes,tooltip];
	            }
	      });
      }
 });