Bubble Sort Algorithm in DSA Javascript

Mental Model

Bubble sort compares pairs of neighbors and swaps them if they are in the wrong order, pushing the largest unsorted value to the end each pass.

Analogy: Imagine bubbles rising in water: the biggest bubble slowly moves up to the surface by swapping places with smaller bubbles below it.

Array: [5, 3, 8, 4, 2]
Indexes:  0   1   2   3   4

Dry Run Walkthrough

Input: list: [5, 3, 8, 4, 2]

Goal: Sort the list in ascending order using bubble sort

Step 1: Compare 5 and 3, swap because 5 > 3

[3, 5, 8, 4, 2]

Why: Swap to move larger number rightwards

Step 2: Compare 5 and 8, no swap because 5 < 8

[3, 5, 8, 4, 2]

Why: Numbers in correct order, no change

Step 3: Compare 8 and 4, swap because 8 > 4

[3, 5, 4, 8, 2]

Why: Move larger number 8 rightwards

Step 4: Compare 8 and 2, swap because 8 > 2

[3, 5, 4, 2, 8]

Why: Largest number 8 bubbles to the end

Step 5: Start second pass: compare 3 and 5, no swap

[3, 5, 4, 2, 8]

Why: First two numbers in order

Step 6: Compare 5 and 4, swap because 5 > 4

[3, 4, 5, 2, 8]

Why: Move 5 rightwards

Step 7: Compare 5 and 2, swap because 5 > 2

[3, 4, 2, 5, 8]

Why: Move 5 rightwards

Step 8: Start third pass: compare 3 and 4, no swap

[3, 4, 2, 5, 8]

Why: Numbers in order

Step 9: Compare 4 and 2, swap because 4 > 2

[3, 2, 4, 5, 8]

Why: Move 4 rightwards

Step 10: Start fourth pass: compare 3 and 2, swap because 3 > 2

[2, 3, 4, 5, 8]

Why: Move smaller number leftwards

Result:

Final sorted array: [2, 3, 4, 5, 8]

Annotated Code

DSA Javascript

class BubbleSort {
  static sort(arr) {
    let n = arr.length;
    for (let i = 0; i < n - 1; i++) {
      // Each pass pushes largest unsorted to end
      for (let j = 0; j < n - 1 - i; j++) {
        if (arr[j] > arr[j + 1]) {
          // Swap if left is bigger
          let temp = arr[j];
          arr[j] = arr[j + 1];
          arr[j + 1] = temp;
        }
      }
    }
    return arr;
  }
}

// Driver code
const input = [5, 3, 8, 4, 2];
const sorted = BubbleSort.sort(input);
console.log(sorted);

for (let i = 0; i < n - 1; i++) {

outer loop controls number of passes

for (let j = 0; j < n - 1 - i; j++) {

inner loop compares adjacent pairs, shrinking range each pass

if (arr[j] > arr[j + 1]) {

compare neighbors to decide if swap needed

let temp = arr[j]; arr[j] = arr[j + 1]; arr[j + 1] = temp;

swap to move larger number rightwards

Output

[2, 3, 4, 5, 8]

Complexity Analysis

Time: O(n^2) because each element is compared multiple times in nested loops

Space: O(1) because sorting is done in place without extra storage

vs Alternative: Compared to more advanced sorts like quicksort (O(n log n)), bubble sort is slower but simpler to understand

Edge Cases

empty array []

returns empty array immediately without error

DSA Javascript

for (let i = 0; i < n - 1; i++) {

array with one element [7]

returns the same single-element array unchanged

DSA Javascript

for (let i = 0; i < n - 1; i++) {

array with all equal elements [2, 2, 2]

no swaps occur, returns same array

DSA Javascript

if (arr[j] > arr[j + 1]) {

When to Use This Pattern

When you see a problem asking to sort by repeatedly swapping neighbors until sorted, think of bubble sort because it pushes largest values to the end step-by-step.

Common Mistakes

Mistake: Not reducing inner loop range each pass, causing unnecessary comparisons
Fix: Use 'n - 1 - i' as inner loop limit to avoid checking already sorted tail

Mistake: Swapping incorrectly or forgetting to swap neighbors
Fix: Always swap arr[j] and arr[j+1] when arr[j] > arr[j+1]

Summary

Bubble sort repeatedly swaps adjacent elements to sort an array in ascending order.

Use bubble sort for small or nearly sorted lists when simplicity matters more than speed.

The key insight is that each pass moves the largest unsorted element to its correct position at the end.