Rufars

let currMonth = new Date().getMonth()
let monthList = ['jan', 'feb', 'mar'] // ...
let currMonthName = monthList[currMonth]

$('#'+currMonthName).addClass('active')
// ...

var months = ['jan','fev','mar','apr','may','iyun'],
	month = new Date().getMonth();

if(months[month]) {
	document.querySelector("a[href='#"+months[month]+"']").parentNode.classList.add("active");
	document.getElementById(months[month]).classList.add("in", "active");
}