JSFiddle - React, Tailwind, and code Playground

JavaScript

var existing_names = ["username1", "username2", "username3"];

var new_names = ["username1", "username4", "username5"];

/* http://snook.ca/archives/javascript/testing_for_a_v */ 

function oc(a)
{
  var o = {};
  for(var i=0;i<a.length;i++)
  {
    o[a[i]]='';
  }
  return o;
}


for (var i=0; i < existing_names.length; i++) {
    if (!(existing_names[i] in oc(new_names))) {
        new_names.push(existing_names[i]);
    }
}

console.log(new_names);