JSFiddle - React, Tailwind, and code Playground

JavaScript

var arr1 = [123, 456, 789], arr2 = [123, 456, 789, 098];

function toObject(arr){
    return arr.reduce(function(o, v, i) {
      o[v] = i;
      return o;
    }, {});
}

var o1 = toObject(arr1), o2 = toObject(arr2), diff = [];

for(var prop in o2){
    if(!o1.hasOwnProperty(prop))
        diff.push(prop);
}

console.log(diff);