How to implement merge sort in JavaScript?

JavascriptObject Oriented ProgrammingProgramming


Merge sort is an example of a divide-and-conquer type sorting-algorithm.The input of merge sort is an array of some elements, which are need to be arranged typically from least to the greatest. 

Steps to follow in Merge Sort

  • Merge sort divides the array in to two sub arrays and later divides each array in to another two arrays and so on until a bunch of single element arrays are left. For instance, in the following example the array [4,7,5,9,1,3,8,2] divides in to single array elements such as [4], [7], [5], [9], [1], [3], [8], [2].
  • It starts comparing arrays in such a manner that two arrays are compared and concatenated. In the following example, it compares two arrays at a time that is  [4], [7] are compared and concatenated then [5], [9] are compared and concatenated and so on such that arrays [4,7], [5,9], [1,3], [2,8] are formed.
  • It follows the same way that is two-two arrays are compared and concatenated to form two arrays. In the following example [4,7]  and [5,9] are compared and concatenated to get an array as [4,5,7,9] and same is the case with other two arrays to form an array as [1,2,3,8].
  • Same rule applicable here that is the remaining two arrays compares and concatenates to get a final array as [1,2,3,4,5,7,8,9].


Live Demo

   function mSort (array) {
      if (array.length === 1) {
      return array                            // return once we hit an array with a single item
   const middle = Math.floor(array.length / 2) // get the middle item of the array rounded down
   const left = array.slice(0, middle)         // items on the left side
   const right = array.slice(middle)           // items on the right side
   return merge(
   // compare the arrays item by item and return the concatenated result
   function merge (left, right) {
      let result = []
      let leftIndex = 0
      let rightIndex = 0
      while (leftIndex < left.length && rightIndex < right.length) {
         if (left[leftIndex] < right[rightIndex]) {
         } else {
   return result.concat(left.slice(leftIndex)).concat(right.slice(rightIndex))
   const list = [4,7,5,9,1,3,8,2]


Published on 31-May-2019 15:04:00