TL;DR
If you haven’t already, learn one of the popular JS frameworks: React, Angular, Vue, etc.
Read this instead: A Review of Udacity’s React Nano Degree
If you aren’t using one of these post-modern frameworks yet, take some time to dig into it a bit. There are plenty of tutorials.
However you do it, do it.
From the job listings I’m seeing all up in my inbox, you need to know a framework and you need a design/developer portfolio. In other words, have side or exploratory projects or code samples of public-facing web sites, applications, or github repos.