var arr = [1, 3, 2, 1, 2, 3, 2, 1]
    var l = arr.length;

    for (var i = 0; i < l; i++) {
        if (arr[i] === 1) {
            arr.splice(i, 1)
            arr.unshift(1)
        } else if (arr[i] === 3) {
            arr.splice(i, 1)
            arr.push(3)
        }
    }

    [1, 1, 1, 2, 2, 2, 3, 3]

var array = [3, 3, 2, 1, 3, 2, 1],
    MID = 2,
    i = 0,
    j = 0,
    n = array.length - 1;

while (j <= n) {
    if (array[j] < MID) {
        [array[i], array[j]] = [array[j], array[i]];
        i++;
        j++;
    } else if (array[j] > MID) {
        [array[n], array[j]] = [array[j], array[n]];
        n--;
    } else {
        j++;
    }
}

var arr = [1,2,3,4]
[arr[0], arr[1]] = [arr[2], arr[3]]