String.prototype.rotate from @js_tut challenge

rotate.js

/* https://twitter.com/js_tut/status/1262754711502942213 */

String.prototype.rotate = function(shiftVal){
	var str = this.toLowerCase();
	var newPos = shiftVal > 0 ? (str.length - (shiftVal % str.length)):(Math.abs(shiftVal) % str.length);
	if(newPos == str.length) 
		return str.charAt(0).toUpperCase() + (str.length > 1 ? str.substring(1,str.length):'');
	return str.charAt(newPos).toUpperCase() + str.substring(newPos+1,str.length) + str.substring(0,newPos);	
}