JSFiddle - React, Tailwind, and code Playground

JavaScript

var arr1= [1, 2, 5, 9], arr2 = [2, 5, 100, 0, 41];

function getUniqCount(arr1, arr2) {
  var allElements = arr1.concat(arr2);
  return allElements
    .filter(el => appearsOnce(allElements, el))
    .length;
}

function appearsOnce(array, what) {
    return array.filter(item => item == what).length === 1;
}

console.log(getUniqCount(arr1, arr2));