JavaScript errors are most often (almost always) caused by plugin conflicts, so to begin troubleshooting this issue you should deactivate all plugins (all at the same time) to make sure the plugins you have installed are not causing any issues. If the problem goes away, reactivate your plugins one at a time (individually, not incrementally) until you find the one responsible and let us know. If the problem persists after all plugins are deactivated (all at the same time), let us know and we will work from there.