i know this is an old post but hopefully this will help someone.
In the 1990’s different versions of js started coming out like javascript from netscape, Js script from Microsoft. So ecmascript was introduced as a standard. But ecmascript forms only a part of javascript which specifies its core syntax,types,objects etc. Probably that explains the inconsistent implementations of javascript across diff. browsers
Reference - Wrox(Professional Javascript For Web Developers)