after completing the course on javascript What do I need to do next ?

completed half of the course on java script
But after these courses, where should you apply your knowledge? What should you do in further development? What did you usually do after these courses?

you can try to develop something using vanilla JavaScript, (you would need to look up on your own DOM manipulation, which allows JS to interact and control HTML elements), or you can go forward with the other certificates - React, Node, Express that you will meet later, that is all JS, so you will keep practicing it

I am a little over halfway through on this course and I created a To Do List application that helps me track my goals and achievements throughout the day.

after learning the react?

A dummy product of my knowledge gained through responsive web design course combined with the basic javascript.