Skip to content

JavaScript Algorithms: Quicksort

New Course Coming Soon:

Get Really Good at Git

Quicksort is a more efficient searching algorithm than selection sort, in most cases, and it makes use of recursion.

Recursion means we call a function from within the same function. It’s a very useful practice, sometimes, and this is one of those cases.

I said “in most cases”, because as we’ll see, in the worst case bubble sort can take the same time of selection sort: O(n^2). But in the best case scenario, it will run at O(n log n), which is in the middle between O(n) and O(n^2).

How does it work? Given an array, we pick an item, called pivot. We then get all the items smaller than the pivot, and the items bigger than the pivot.

Then we run the same operation on the 2 array that compose the smaller and bigger items.

It’s easier to see the code than to describe it:

const quickSort = (originalList) => {
  const list = [...originalList]

  if (list.length < 2) {
    return list
  }

  const pivot = list[0]

  const smaller = list.filter((item) => item < pivot)
  const bigger = list.filter((item) => item > pivot)

  return [...quickSort(smaller), pivot, ...quickSort(bigger)]
}

In this case I chose the pivot to be the first item in the array, but it could also be the item in the middle, for example:

const pivot = list[Math(floor(list.length / 2)]

Notice how we first copy the array, so calling quickSort() does not modify the original array, it just returns a new sorted array:

const a = [1, 6, 3, 4, 5, 1, 0, 4, 8]

console.log(quickSort(a))
//[0, 1, 1, 3, 4, 4, 5, 6, 8

console.log(a)
//[1, 6, 3, 4, 5, 1, 0, 4, 8]
Are you intimidated by Git? Can’t figure out merge vs rebase? Are you afraid of screwing up something any time you have to do something in Git? Do you rely on ChatGPT or random people’s answer on StackOverflow to fix your problems? Your coworkers are tired of explaining Git to you all the time? Git is something we all need to use, but few of us really master it. I created this course to improve your Git (and GitHub) knowledge at a radical level. A course that helps you feel less frustrated with Git. Launching May 21, 2024. Join the waiting list!
→ Get my JavaScript Beginner's Handbook
→ Read my JavaScript Tutorials on The Valley of Code
→ Read my TypeScript Tutorial on The Valley of Code

Here is how can I help you: