quick sort using filter in JavaScript

Question

I am working on quick sort using recursion and filter, but the recursion is not working correctly. It returns to the first half of the list sorted + extra (pivot and second half of the list from the last recursion), but second half of the list disappears. Here is my code.

 const { List } = require('immutable') const quicksort = function(list) { if (list.size <= 1) { return list; } pivot = list.last(); part1 = list.pop().filter((x) => x <= pivot); part2 = list.pop().filter((x) => x > pivot); return quicksort(part1).concat(list.last(), quicksort(part2)); }; console.log("quicksort: " + quicksort(List([4, 7, 3, 6, 8, 7, 1, 2, 2, 1, 5]))); 

and it prints out

quicksort: List [ 1, 1, 2, 3, 3, 3, 5 ]

The first time quicksort is called,

part1 is [ 4, 3, 1, 2, 2, 1 ]

pivot is 5

part2 is [ 7, 6, 8, 7 ]

and basically, [ 7, 6, 8, 7 ] just disappears.

I appreciate any insight and advice. Thank you!

Answer 1

var , let or const declaration is missing. Without those, pivot , part1 and part2 are global variables . After sorting part1 , part2 has been overwritten to an empty list (because that was the terminating circumstance of the previous branch of the recursion). Just let your vars be properly local :p. Like this:

let pivot = list.last();
let part1 = list.pop().filter((x) => x <= pivot);
let part2 = list.pop().filter((x) => x > pivot);

Lesson : "use strict";