How to upload static json file into indexedDB ONLY when an upgrade is needed (i.e. onupgradeneeded) - xmlhttprequest

Question

I'm tyring to upload a static json file into an indexedDB ONLY when an upgrade is needed (i.e. onupgradeneeded). I've search for answers to this repeatedly but have yet to see code examples of how to approach this.
My current code below gets the json file every time the page opens, which is of course inefficient since I only need to get the json file if the indexedDB has not yet been created or needs upgraded.
I tried putting the xhr.onload section into the end of the .onupgradeneeded function, but as many have noted, the .onsuccess gets called before the xhr.onload has completed.
var jsonUrl = '/path/to/hcLookup.json');
var req, db, hcObjectStore, objectStore, data, dataArr, trans, addreq, key;
var xhr = new XMLHttpRequest();
xhr.open("GET", jsonUrl, true);
xhr.type='json';
xhr.send();
xhr.onload = function(msg) {
data = msg.target.response;
req = window.indexedDB.open("hcLookup", 1);
req.onerror=function(event){console.log("onerror: " + event.target.errorCode)};
req.onsuccess = function(event){
console.log("ready.");
};
req.onupgradeneeded = function(event){
db = event.target.result;
objectStore = db.createObjectStore("hcLookup", {autoIncrement: true});
objectStore.createIndex("S", "S", {unique: false});
// make sure the objectStore creation is finished before adding data into it
objectStore.transaction.oncomplete = function (event) {
// Store values in the newly created objectStore.
trans = db.transaction(["hcLookup"], "readwrite");
hcObjectStore = trans.objectStore("hcLookup");
// Do something when all the data is added to the database.
trans.oncomplete = function (event) {
console.log("upgrading done!");
};
trans.onerror = function (event) {
console.log("bulk add onerror: " + event.target.errorCode)
};
//convert JSON to an strArray in order to add the dataArr into to the objectStore
dataArr = JSON.parse(data);
for (var i in dataArr) {
addreq = hcObjectStore.add(dataArr[i]);
}
};
};
};