mergesort.js

const mergeModule = ( () => {
  // used to merge all of our pieces back together after recursively separating the array
  const merge = ( left, right ) => {
    // initialize array to return
    const result = [];

    // if both of our split arrays have items inside go through this while loop
    while ( left.length > 0 && right.length > 0 ) {
      // compare the first element of each array
      if ( left[0] <= right[0] ) {
        // if the left element is smaller, push it
        // to our return array
        result.push( left.shift() );
      } else {
        // if the right element is smaller, push it
        // to our return array
        result.push( right.shift() );
      }
    }

    // if only our left array has an element left, push that
    while ( left.length > 0 ) {
      result.push( left.shift() );
    }

    // if only our right array has an element left, push that
    while ( right.length > 0 ) {
      result.push( right.shift() );
    }

    // return the sorted array
    return result;
  };

  return {
    mergeSort: function ( arr ) {
      // Base Case - if the array is length 0 or 1,
      //  then we can assume it is already sorted and return it
      if (arr.length < 2) {
        return arr;
      }

      // pick a pivot at our the middle of our arr
      const pivot = ( Math.floor(arr.length / 2) );

      // separate the arr into two places, everything before it
      const pLeft = arr.slice( 0, pivot );

      // and everything after
      const pRight = arr.slice( pivot, arr.length );

      // call our mergeSort recursively on this array,
      //  splitting it further and further until it hits
      //  our base case and the array is split into lengths less than 2
      return merge( this.mergeSort( pLeft ), this.mergeSort( pRight ));
    },
  };
});

module.exports = mergeModule;