The code below here calls `finishCallback` without checking to see if its defined.  If its required, it might be nice to fast-fail up front here:

```javascript
if (typeof finishCallback !== 'function') {
  throw new Error('SIM export requires a callback function');
}
```

Or alternatively we could check at each call site or do something like this:

```javascript
  finishCallback = (typeof finishCallback === 'function') ? finishCallback : function() {};
```