Stop 21 · Data Structures & AlgorithmsTopics · 16/17

Sorting

In short

You'll rarely write quicksort at work, but interviewers still ask how sorting works, what it costs, and when sorting first makes the rest of a problem easy.

Algorithms for ordering data (bubble, merge, quick, etc.) — foundational CS knowledge, and the basis for many other algorithms' efficiency assumptions.

The purple blob arranges books on a shelf from shortest to tallest.

Think of it like this

Arranging a bookshelf

Sorting a messy shelf by height, you might split it into two halves, arrange each half, then merge them by always picking the shorter front book. That's merge sort, and it's how you'd actually do it without losing your mind.

Watch it happen

Merge sort on [8, 3, 7, 1]

1/4Split the array into halves: [8, 3] and [7, 1].

The big ideas

What you need to know

O(n log n) is the target

Merge sort, heap sort and quicksort on average all take O(n log n). Bubble and insertion sort are O(n²).

Merge sort is stable

Equal items keep their original order. Useful when you sort by one field after another.

Quicksort picks a pivot

Smaller items go left, bigger go right, then recurse. Fast in practice, but bad pivots make it O(n²).

JS sort compares text

Without a comparator, [10, 9].sort() treats numbers as strings. Always pass (a, b) => a - b.

See it in code

Walk through the code

1const marks = [100, 25, 9, 40];2marks.sort();                    // [100, 25, 40, 9]  (compared as text!)3marks.sort((a, b) => a - b);     // [9, 25, 40, 100]4 5const students = [6  { name: "Riya", cgpa: 8.9 },7  { name: "Aman", cgpa: 9.4 },8  { name: "Zoya", cgpa: 8.9 },9];10// Highest CGPA first; ties keep their original order (sort is stable)11students.sort((a, b) => b.cgpa - a.cgpa);12// Aman, Riya, Zoya13 14// A sorted copy that leaves the original alone (ES2023)15const topFirst = marks.toSorted((a, b) => b - a);

1/3The default sort turns numbers into strings. A comparator fixes it.

Array.prototype.sort is guaranteed stable since ES2019, so Riya stays ahead of Zoya when their CGPAs tie.

Save yourself the pain

Common mistakes

Oops

Calling .sort() on numbers with no comparator.

Do this instead

Always write .sort((a, b) => a - b). The default compares strings, so 100 comes before 25.

Oops

Forgetting that sort() changes the original array.

Do this instead

Copy first with [...arr] or use toSorted() if other code still needs the original order.

Your mission

Sort smart

~40 min

Quick check

What does [10, 9, 100].sort() return in JavaScript?

Go deeper

1Implement merge sort and quicksort from scratch, not just call a built-in.
2Learn the time/space complexity trade-offs between the common sorting algorithms.
3Understand why most languages' built-in sort is a hybrid algorithm, not a single 'pure' one.