Add Index to Pre-Existing ObjectStore In IndexedDB Using Javascript

I have seen multiple JavaScript examples of using createIndex to define an ObjectStore index directly after the ObjectStore has been created like this:

var objectStore = ixDb.createObjectStore(osName, { keyPath: pkName, autoIncrement: autoIncrement });

objectStore.createIndex("name", "name", { unique: false }); 

Can anyone show me how you would use createIndex on a pre-existing table without calling createObjectStore? I guess the real question here is how to get a reference to the objectStore without using createObjectStore?

I tried several variations of the following with no luck:

var objectStore = window.IDBTransaction.objectStore(ObjectStoreName);
var index = objectStore.createIndex(ixName, fieldName, { unique: unique, multiEntry: multiEntry });