There’s no doubt that front-end development will be one of the hottest disciplines in tech in 2020

7. Testing

No untested code should find it’s way to production.
While it might seem convenient to not have any tests in your personal projects, it’s mandatory to have them when working in commercial and enterprise environments. So it’s better for any developer to integrate tests in the development workflow whenever possible.
One can differentiate between test cases like:

Unit tests

Testing a single component or function in isolation.

Integration tests

Testing interactions between components.

End-to-end test

Testing full-blown user flows in the browser.
There are more ways of testing, such as manual testing, snapshot testing, etc. If you want to move to a senior developer position or aim for a job with a large corporation that has some development standards, you should try to work on your testing skills.

8. Clean Code

Being able to write clean code is a great skill and is in high demand by many organizations. If you want to step up from a developer position to a senior developer position, you should really learn the concepts of clean code.
Clean code should be elegant and pleasing to read. It should be focused, and you should take care of it. All tests are run in clean code. They shouldn’t contain duplications, and the use of entities, such as classes, methods, and function, should be minimized.
Some things a clean-code developer should do are:

• 
  Create meaningful names for variables, classes, methods, and functions
  
• 
  Functions should be small and have as few arguments as possible
  
• 
  Comments shouldn’t be required at all — the code should speak for itself
  

If you want to learn more about clean coding check, out the books and posts from Robert C. Martin.