How can I fully understand the Javascript?

Answer 1

You can learn javascript from sololearn easily

Answer 2

All you need is practice.
I will be honest, Sololearn barely scratches the surface, there are wonderful other sources to learn.
Remember, it won't be easy, but that's the reason developers get high pay. Here are some resources you can use:

The best place to start - https://javascript.info/

Watch this to get a overview of what you will be learning - https://youtu.be/DFs-du7Uc2w

I used this to when I was getting started with JS - https://youtu.be/Xm4BObh4MhI
It is long but worth it.

Course on FreeCodeCamp - https://www.freecodecamp.org/learn/javascript-algorithms-and-data-structures/

JavaScript terminology - http://jargon.js.org/