Quick Select. Also can be termed as Finding Kth Smallest Element in Unsorted array with no duplicates.

QuickSelect.js

function swap(arr, i, j) {
    var temp = arr[i]
    arr[i] = arr[j]
    arr[j] = temp
    return arr
}

function partition(arr, start, end) {
    var pivot = end
    while(start <= end) {
        if (arr[start] < arr[pivot]) {
            start++
        } else if (arr[end] > pivot) {
            end--
        } else {
            swap(arr, start, end)
            start++
            end--
        }
    }
    swap(arr, start, pivot)
    return start
}

function quickSort(arr, start, end) {
    if (end > start) {
        var newPivot = partition(arr, start, end)
        quickSort(arr, start, newPivot-1)
        quickSort(arr, newPivot+1, end)
    }
}

function quickSelect(arr, k, start, end) {
    if (end > start) {
        var pivot = partition(arr, start, end)
        if (pivot === k-1) {
            return arr[pivot]
        } else if (k-1 < pivot) {
            return quickSelect(arr, k, start, pivot-1)
        } else if (k-1 > pivot) {
            return quickSelect(arr, k, pivot+1, end)
        }
    }
}

function select(arr, k) {
  quickSelect(arr, k, 0, arr.length-1)
}

function findKthSmallestElement(arr, k) {
  quickSelect(arr, k, 0, arr.length-1)
}