In that regard JavaScript is a lot like C++. You can write pretty good C++ if you know what you are doing but the old ways are still possible and haunt you in legacy code.
However, with JavaScript you have to avoid the old/outdated language constructs, and some warts need to remain, since browsers still need to support old websites. Perhaps in the future we can specify "ECMAScript version" (similar to perl) so old stuff can be thrown away and the VM can optimize without having to worry as much about backwards compatibility.