Like what? I hear this argument all the time, but it's always unsubstantiated.
In my experience, the choice of language is less important than the programmer's understanding of facts that have little to do with the language itself.
If you are developing client-side browser code, for instance, then you have little choice but to use Javascript.
But learning JavaScript is not the hard part: mainstream programming languages, in and of themselves, are rarely difficult to master.
On the contrary, the skill that separates the superior engineers from the inferior ones is the mastery of the environment that the program operates in.
For front-end developers, it is an intricate understanding of how browsers work (i.e., the DOM and supported events) and some good understanding of how to optimize requests back to the server. (There is probably much more they know than this, but I'm not a front-end developer.)
Similarly, back-end developers have much to learn about how to efficiently and reliably serve (hopefully) many thousands of requests per second. They need to understand the limitations of their servers, how to manage memory and storage effectively, and build reliable, operable services.
For both types, these skills often take many years to master, and they are hardly interchangeable. Consider that developers themselves often describe themselves as "front-end" and "back-end" people. The fact that you can now use JavaScript on the server side does little to disturb this reality; and things wouldn't change much if you could run Perl or Python or Ruby in the browser tomorrow.