let arr1 = [2,2,2,5,5,6,6,6,6,7];
function trans (arr) {
    let pre = NaN
    let result = []
    let curArr
    arr.forEach(cur => {
        if (cur === pre) {
            curArr.push(cur)
        } else {
            curArr = [cur]
            result.push(curArr)
            pre = cur
        }
    })
    return result
}
trans(arr1)

[2,2,2,5,5,6,6,6,6,7]
.reduce((m,n) => (m[n] = (m[n] || []).concat(n), m),[])
.filter(a => a)

你的一維數組是否已經排好序？
如果已經排好，處理比較簡單，否則會比較麻煩。