The answer to your question all depends on exactly what kind of jobs you are even remotely interested in?

In general it's best to compound your current skills and not go in a completely perpendicular direction to where you are going now.

Given that: - you are good in JavaScript - you haven't provided much filter of criteria - you mentioned a curiosity in machine learning - you care about job prospects and want a skill that will stay relevant

Have you considered data visualization? If you become a master at that, you can break into the higher level parts of machine learning/data science that require visualizing results to end-users/interested stakeholders. I'm good enough with D3.js to understand examples and create my own modified graphs, but I wish I had the skills to recreate all of the cool visuals you can see in their gallery.