This is a little challenge another developer hit me with. It will return the first letter after the first set of repeated letters in a string. For example, in aardvark it will return the 'r' after 'aa'. It will also return the letter in the correct case.

aardvark.js

function aardvark(str){

  var len = str.length, tempStr, iteration;
	
	// if str is 2 characters or less return null right away
	if(len < 3) return null;
	
	// temporarily convert str to all lowercase to properly compare each character
	tempStr = str.toLowerCase();

	// if string length is even iterate length - 2, else length - 1
	if(len % 2 === 0){
		iteration = len - 2;	
	}else{
		iteration = len - 1;
	}
	
	// iterate over tempStr to compare all characters equally
	for (var i = 0; i < iteration; i++) {
		
		// if two adjacent characters match return the following character from the original string to preserve case	
		if(tempStr[i] === tempStr[i+1]){
			
			// if str is only 3 characters and the last 2 match (ex. 'add') this will attempt to
			// return the 4th character which doesn't exist. instead of returning undefined we want
			// to catch this and explicitly return null
			if(!str[i+2]) return null;
			
			// return character from original string to preserve case
			return str[i+2];		
		}
		
	}
	
	// if we've made it this far there were no repeating characters, or the only repeating characters were at the end of the string
	return null;
	
}



//console.log(aardvark("aardvark"));
//console.log(aardvark("little"));
//console.log(aardvark("craddles"));
//console.log(aardvark("LiTtLe"));
//console.log(aardvark("Carlin"));
//console.log(aardvark("Barrero"));
//console.log(aardvark("BarrEro"));
//console.log(aardvark("tall"));
//console.log(aardvark("no 'x' 120044()[]\"{}<>\/$~^|-+%*=!?#&@_:;.,''innixon"))
//console.log(aardvark("bb"));
//console.log(aardvark("bba"));
//console.log(aardvark("abc"));
//console.log(aardvark("add"));
//console.log(aardvark("llat"));