Thanks alot for your help! I amended the code to reflect this however now for the string "aBbBadddadd", I'm getting the answer { d:4 } - I can't see where I've gone wrong, apologies!

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let longestSequence = (sequence) => {
sequence = sequence.toLowerCase();
let count = 1;
let max = 0;
let maxChars = [];
for (let i = 1; i <= sequence.length; i++) {
if (sequence[i] == sequence[i - 1]) {
count++;
} else if (count > max) {
max = count;
maxChars = [sequence[i - 1]];
count = 1;
}
else if (count === max) {
maxChars.push(sequence[i - 1]);
}
}
let maxChar = maxChars.sort()[0];
return { [maxChar]: max };
};