Doing non trivial stuff will be the challenge, not the programming language. With that said I think Javascript/front end is more of a moving target and you will

Doing non trivial stuff will be the challenge, not the programming language. With that said I think Javascript/front end is more of a moving target and you will constantly need to relearn the same things using new frameworks etc. In something like PHP or Perl you can get more years of relevant knowledge from your effort. But some people like shiny things that break often ;) If JS didn't have a monopoly in browsers I think something else would have been more popular. Back end is definitively a deeper area once you are past beginner/intermediate level.