function repeat(arr) {
    let counter = 0,
        value;
    for (let i = 0; i < arr.length; i++) {
        if (arr[i] === arr[i + 1]) {
            counter++;
            value = arr[i];
        }
    }
}[1,2,3,4,4,4, 4,5,2,1,2] = > 4  function repeat(arr) {
	let repeats = {},
		maxValue,
		maxIter;
	for (let i = 0; i < arr.length; i++) {
		let value = arr[i];
		if (repeats[value] === undefined)
			repeats[value] = 0;
		++repeats[value];
		if (maxValue === undefined
			|| repeats[value] > maxIter) {
			maxValue = value;
			maxIter = repeats[value];
		}
	};
	return maxValue;
}