Array Unique Count only sorted Array

function UniqueCount(arr){
// for(var i,j=0;j<arr.length;)
let i=0
for(let j=1;j<arr.length;j++){
if(arr[i]!=arr[j]){
i++
arr[i]=arr[j]
}
}

return i+1

}

const count=UniqueCount([1,1,2,3,3,3,3,3,3,3,3,3,4,5,5,5,5,6,7,8,8,9])
console.log('count',count)