I need to learn JavaScript. What is the best solution?

Maybe think of a problem that you want to solve via JS, (that depends on you programming experience tho.) for me learning a new language or framework is easiest when having a more ore less concrete idea what i want to achieve. That is an effective way for me.
Otherwise there are many online courses or videos which teach the basics of js.
Imo most of them do a decent job of teaching the basics. Just start coding :D

Here are some suggestions for where to start.

developer.mozilla.org/en-US/docs/W...
codecademy.com/
codewars.com/?language=javascript
egghead.io/search
w3.org/wiki/JavaScript_best_practices
frontendmentor.io/challenges

I think you should start by re-creating things. Back when I was learning my first programming language, I wanted to re-create facebook and instagram, so those were my projects to learn a programming language, it didn't mean I wanted to create a social media or something like that, it just meant that I was re-creating something to learn. When you re-create something to learn, you actually learn through putting your own "flavor", this is because when you are re-creating something (let's say instagram) you also want to personalize it in your own way, and you start adding features that don't exist in the real application, but those kind of things are cool for you and get you learning along the way