Q:

Uncaught InvalidStateError: Failed to execute 'transaction' on 'IDBDatabase': A version change transaction is running

i must admit that i am very new to indexedDB
I wrote a simple code of indexedDB  and it is as followed:
function go() {
   var req = window.indexedDB.open("Uploader", 1),
      db;
   req.onerror = function (e) {
      console.log("Error");
   };
   req.onsuccess = function (e) {
      db = e.target.result;
   };
   req.onupgradeneeded = function (e) {
      console.log(db);
      db = e.target.result;
      var os = db.createObjectStore("Files", { keyPath: "files" });
      os.createIndex("text", "text_file", { unique: false });
      var trans = db.transaction(["text"], "readwrite");
      var objectstore = trans.objectStore("text");
      var addreq = objectstore.add("Instructions.js");
      addreq.onsuccess = function (e) {
         console.log("Success!");
         console.dir(e);
      };
   };
}

the error it is giving me is Uncaught InvalidStateError: Failed to execute 'transaction' on 'IDBDatabase': A version change transaction is running.
It is saying that A version change Transaction is running but as far as i have studied, a version change transaction is made from IDBFactory.open method and i haven't used and i have specified that this transaction is readwrite and this transaction is in onupgradeneeded then why there is an error?
i must admit that i am very new to indexedDB

A:

The versionchange transaction also allows you to readwrite. You just need to access the transaction created for you within the onupgradeneeded function.
function go() {
  var req = indexeddb.open(...);
  req.onupgradeneeded = function(event) {

    // note that event.target === req === this, use whatever you like
    var db = event.target.result;

    // createObjectScore implicitly uses the versionchange txn running 
    // here, without telling you, basically a convenience function
    var objectStore = db.createObjectStore(...);

    // the important part that i am describing in this answer, 
    // grab the handle of the versionchange txn 
    // that is already running and was created for you
    // note again that event.target === req, you could just do
    // req.transaction or this.transaction here.
    // note this is a property of the open request, not a method. do NOT
    // confuse this with the method transaction() that is used to create a 
    // new transaction.
    var txn = event.target.transaction;

    // note that txn.objectStore(...) will work here, because the 
    // createObjectStore call earlier guarantees the store exists here
    // within the implicit upgrade txn
    var addRequest = txn.objectStore(...).add('value');

    // side note: if in previous line we did:
    // var objectStoreRetrievedFromTxn = txn.objectStore(...);
    // then that variable is equal to the same variable returned from 
    // db.createObjectStore earlier in this function. Both are simply handles 
    // (references, pointers, whatever you want to call it) to the store.

    // kind of dumb, but you could do this just to log something
    addRequest.onsuccess = function() {console.log('Success!');};
  };

  // called once upgrade txn completes (if it even needed to run), 
  // and db is opened
  req.onsuccess = function(event) {
    console.log('upgrade txn completed and db is now connected');
    // here, create whatever readwrite or readonly txns you want, note these 
    // txns are separate from and different than the versionchange txn from 
    // before, because that is a unique transaction only available within 
    // onupgradeneeded. however, both versionchange and readwrite are similar
    // in that both support calls to put or add
  };
}

You are encountering the error because you are trying to start a second transaction while the version change transaction is still running.

A:

You need to check for the completion of the version change transaction before attempting to load the object store:
request.onupgradeneeded =
    function(event) {
        db = event.target.result;
        var store = db.createObjectStore('Files', {keyPath:"files"});
        var transaction = event.target.transaction;

        transaction.oncomplete =
            function(event) {    
                // Now store is available to be populated
            }
    }

