What to read for "mediocre" js developer?

learn.javascript.ru

ie the fundamental concepts of know, work with it , but caught some example so you don't know what to do

It's called "ponahvatalsya here and there, but entirely so nothing learned". View some screencast completely. Try to write something more difficult, "helloworld'a".

If you want to become an engineer with a capital letter, then be sure to check SICP. The book for which it does not matter what you write now, it gives us a deep understanding of code design principles and creates the right mindset. In General, the list of recommended and not language specific https://ru.hexlet.io/pages/recommended-books

From the books which use js as the primary language, you can try https://github.com/MostlyAdequate/mostly-adequate-guide (all the same is in Sike at a more fundamental level). The book has a sort of translation into Russian.

A good look at the language in any good textbook from cover to cover, read articles, learn of typical tasks on stackoverflow, study someone else's code(for example repository on Github), write your much.
Best progress will go after the device to work.

Read more about how to independently and effectively find the information they need. One of the most useful skills.

100% of the sites I've seen use jQuery. I have in my time used this: https://oscarotero.com/jquery/, direct take all the methods and read about each to know what's going on. You can try a couple of plugins for jQuery like any slider (jCarousel, Slick), photo galleries (well, it's all just a lightbox). With jquery plugins it's simple really, the main thing to learn them to "enjoy".

But modern javascript is react, nodejs, etc if you want to be web full stack js. I'm not strong because I am a php programmer, but in General, if English is, then just take a course (just looking at the tracker 'react', 'nodejs') and look.

UPD:
And, Yes, pure JS is a good course learn.javascript.ru, "Main course": part one and some things from part two. This and some books on JS enough to perform normal operations.

Enough David Flanagan read to understand JS (EcmaScript 5) thoroughly:
\rhttps://m.vk.com/wall-54530371_2706

Only EcmaScript 6 (+7+8...) after Flanagan also add to the knowledge you will need.

A programming language is a tool, like a hammer or a food processor.

Any tool can cause good and cause benefit and harm. But most often, the tool is just gathering dust on the shelf. Same tool different masters done differently. I.e. it all comes down to the master, his skills, character, perseverance, tenacity, perfectionism, in the end of it all.

It is clear that tool it is advisable to possess masterly. But one instrument strongly enough. It is foolish to assume that you learned JS you can conquer the peaks. For a start has a considerable number of those engaged in technology, but at least the same HTML+CSS, DOM, browsers with their nuances and API, HTTP, Ajax, Rest, algorithms, data structures, storage, and million just.

Any project consists of a huge set just to get acquainted with the main ingredients at least at the top will take thousands of man-hours enough dedicated work.

In General advice here, this is already given, above, but not explained why. So, be stupid to do. To get hand, so to speak. As making have with all this zoo, anyway, to get to know. In about a thousand hours already naked the rest of the basic patterns, what to what, how, and why. The first would be "Well, finally did, neponyatnaya", and that's fine. Decomposing large and complex to smaller and not so scary, and doing what they can. Decomposers rest again and so on. And Google, gulim, Google, Google, Google... Well, you understand.

On some days, when I meet another new technology, from 8 hours to 7.5 I Google, read the docks, watching screencasts and other tutorials. Over time I have formed the primary image and the model, and then already pens.

PS: If you do not speak English sufficiently to be free to understand spoken language and texts that I do, in General, do not envy...

Here is a selection of links for training.