Sorting Algorithms with Javascript (Part 1)

October 18, 2018

I've been learning a lot about data structures and algorithms lately and I've noticed in my reading that there aren't a lot of examples showing implementations of algorithms in Javascript. You'll mostly find examples in Java, Python, C, C++ etc. Maybe there's a reason for preferring these languages over Javascript? I'm not sure.

In this first part, I'm going to show Javascript implementations of three sorting algorithms:

  • Merge sort
  • Insertion sort
  • Bubble sort

This is not intended to be an in-depth explanation on the ins and outs of how the algorithms work and their performance. If you'd rather read about that, here's a nice resource I found: Sorting Algorithms

To keep things simple, I'll be sorting a simple list list having only 5 elements [4, 2, 3, 1, 5].

Merge Sort

Merge sort uses a divide-and-conquer approach to sort elements in an array. Basically, what this means is that instead of dealing with the array as a whole, it continually splits it in half until both halves are sorted, then the halves are merged into one solved list.

Visual

merge-sort

Code

function mergeSort(list) {
  const len = list.length
  // an array of length == 1 is technically a sorted list
  if (len == 1) {
    return list
  }

  // get mid item
  const middleIndex = Math.ceil(len / 2)

  // split current list into two: left and right list
  let leftList = list.slice(0, middleIndex)
  let rightList = list.slice(middleIndex, len)

  leftList = mergeSort(leftList)
  rightList = mergeSort(rightList)

  return merge(leftList, rightList)
}

// Solve the sub-problems and merge them together
function merge(leftList, rightList) {
  const sorted = []
  while (leftList.length > 0 && rightList.length > 0) {
    const leftItem = leftList[0]
    const rightItem = rightList[0]
    if (leftItem > rightItem) {
      sorted.push(rightItem)
      rightList.shift()
    } else {
      sorted.push(leftItem);
      leftList.shift()
    }
  }

  // if left list has items, add what is left to the results
  while (leftList.length !== 0) {
    sorted.push(leftList[0])
    leftList.shift()
  }

  // if right list has items, add what is left to the results
  while (rightList.length !== 0) {
    sorted.push(rightList[0])
    rightList.shift()
  }

  // merge the left and right list
  return sorted
}

const list = [4, 2, 3, 1, 5]

const sorted = mergeSort(list)

console.log(sorted)

Insertion Sort

Insertion sort builds the final sorted list one element at a time. It does this by taking one element, comparing it to the rest of elements in the list, finding its right position, and then placing it there.

This is known as comparison-based sorting.

Visual

insertion-sort

Code

function insertionSort(list) {
  const len = list.length
  for (let i = 1; i < len; i++) 
  {
    if (list[i] < list[0]) 
    {
      // move current element to the first position
      list.unshift(list.splice(i,1)[0])
    } 
    else if (list[i] > list[i-1]) 
    {
      // maintain element position
      continue
    } 
    else {
      // find where element should go
      for (let j = 1; j < i; j++) {
        if (list[i] > list[j-1] && list[i] < list[j]) 
        {
          // move element
          list.splice(j, 0, list.splice(i,1)[0])
        }
      }
    }
  }
  return list
}

const list = [4, 2, 3, 1, 5]

const sorted = insertionSort(list)

console.log(sorted)

Bubble Sort

Another example of a comparison-based sorting algorithm, Bubble sort compares each pair of elements in a list and swaps them if they are out of order until the list is sorted.

Visual

bubble-sort

Code

function bubbleSort(list)
{
    let swapped
    let n = list.length-1
    do {
        swapped = false
        for (let i=0; i < n; i++)
        {
            // compare pairs of elements
            // if left element > right element, swap
            if (list[i] > list[i+1])
            {
               const temp = list[i]
               list[i] = list[i+1]
               list[i+1] = temp
               swapped = true
            }
        }
    } 
  // continue swapping until sorted
  while (swapped) 
    
  return list
}

const list = [4, 2, 3, 1, 5]

const sorted = bubbleSort(list)

console.log(sorted)

Thats it! 😊 And, incase you're wondering, I used this site to make the visuals.

In the next part, I'll be going through:

  • Quick sort
  • Heap sort
  • Counting sort