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JavaScript Merge Function that Mergers Two Arrays Already Sorted from Lowest to Highest Numbers
Posted on July 15, 2015 in Algorithms, JavaScript by Matt Jennings

// Function merges 2 arrays that are already sort from
// Lowest to highest numbers
function mergeSortedArrays(arr1, arr2) {
  
  // Empty array that will have rules from 2 arrays
  // pushed into it
  var result = [];
  
  // Length of "result" array that will have 
  // values pushed into
  var i = arr1.length + arr2.length - 1;
  
  // Iterate over the entire length of 
  // arrays "arr1" and "arr2", even if there aren't the same length
  // and the interation continues until "arr1" and/or "arr2"
  // don't have any more elements
  while(arr1.length || arr2.length) {
    // If the same element index from "arr1" is greater than 
    // the same element index from "arr2" pop off that element 
    // from "arr1" and assign it to the last "result" element
    // and decrement by 1
    if(arr1[arr1.length - 1] > arr2[arr2.length - 1]) {
      result[i--] = arr1.pop();
    }  
    // Else the same element index from "arr2" is greater than 
    // the same element index from "arr1" pop off that element 
    // from "arr2" and assign it to the last "result" element
    // and decrement by 1
    else {
      result[i--] = arr2.pop();
    }
  }
  
  // If there are any remaing element from "arr1", 
  // because it doesn't have the same elements as "arr2", 
  // pop off the final element of "arr1" and assign it to 
  // the "result" array and after that decrement by 1
  while(arr1.length) {
    result[i--] = arr1.pop();
  }
  
   // If there are any remaing element from "arr2", 
  // because it doesn't have the same elements as "arr1", 
  // pop off the final element of "arr1" and assign it to 
  // the "result" array and after that decrement by 1
  while(arr2.length) {
    result[i--] = arr2.pop();
  }

  console.log("arr1 elements:", arr1);
  console.log("arr2 elements:", arr2);
  console.log("result array elements:", result);
  
}

var array1 = [1, 2, 5, 9];

var array2 = [-9, 4, 8, 15, 20];

/*
JS console output:
"arr1 elements:"
[]
"arr2 elements:"
[]
"result array elements:"
[-9, 1, 2, 4, 5, 8, 9, 15, 20]
*/
mergeSortedArrays(array1, array2);