JSFiddle - React, Tailwind, and code Playground

JavaScript

function diffArray(arr1, arr2) {
      var newArr = [];
      for (i = 0; i < arr1.length; i++) {
        for (j = 0; j < arr2.length; j++)
          while (arr1[i] === arr2[j])
            arr2.splice(j, 1);
        newArr = arr2;
      }
      return newArr;
    }
function diffArray2(arr1, arr2) {
  let obj1 = {}, obj2 = {};
  for (let l = arr1.length, i = 0; i < l; i++)
  	obj1[arr1[i]] = undefined;
  for (let l = arr2.length, i = 0; i < l; i++)
  	obj2[arr2[i]] = undefined;
  let a = [];
  for (let arr = arr1.concat(arr2), l = arr.length, i = 0, item = arr[0]; i < l; i++, item = arr[i])
  	if (item in obj1 !== item in obj2)
    	a.push(item);
  return a;
}
let arr1 = [], arr2 = [];
for (let i = 0; i < 9999; i++)
	arr1[i] = i;
for (let i = 0; i < 999; i++)
	arr2[i] = i;
let mStart = window.performance.now();
let result1 = diffArray(arr1, arr2);
let m1 = window.performance.now();
let result2 = diffArray2(arr1, arr2);
let now = window.performance.now();
alert((m1 - mStart) + "  " + (now - m1));