given a string with only letters, return the start and end index of the largest consecutive letters

consecStr.js

function check(str){
	var word = str.split("");
	var max = 0;
	var arr1 = [];
	var arr2 = []; 
	var letter = 0;
	var counter = 0; 
	
	for(var i=0; i< word.length; i++){
		if(word[letter] === word[i]){
			counter ++; 
		}else{
			arr1.push(counter);
			arr2.push([letter, i]);
			counter = 0; 
			letter = i; 
			i = i -1;
		}
	}
	console.log(arr1, arr2)
	// find max of array 1
	for(var b=1; b<arr1.length; b++){
		if(arr1[max] < arr1[b]){
			max = b; 
		}
	}
	return arr2[max];
}

var str = "aaannnbbsssnnwwqqqqqqqqqppppwww";
check(str);
//=> [ 15, 24 ]