JSFiddle - React, Tailwind, and code Playground

HTML

<div id="test">

</div>

JavaScript

function updateInventory(arr1, arr2) {

  function alphabetizer(a, b) {
    if (a[1] < b[1]) return -1;
    if (a[1] > b[1]) return 1;
    return 0;
  }

  var newInv = arr1.concat(arr2).sort(alphabetizer);


  for (var i = 0; i < newInv.length; i++) {
    while (newInv[i][1] == newInv[i++][1]) {
      newInv[i] += newInv[i+1][0];
      newInv.push([newInv[i+1][0], newInv[i][1]]);
      newInv.splice(i, 2);
    }
  }

  document.getElementById('test').innerHTML = newInv;
}

// Example inventory lists
var curInv = [
  [21, "Bowling Ball"],
  [2, "Dirty Sock"],
  [1, "Hair Pin"],
  [5, "Microphone"],
  [10, "Apples"]
];

var newInv = [
  [9, "Apples"],
  [2, "Hair Pin"],
  [3, "Half-Eaten Apple"],
  [67, "Bowling Ball"],
  [7, "Toothpaste"]
];

updateInventory(curInv, newInv);