You need to know Javascript's quirks. I generally recommend [1] for experienced developers. Everything on that page is important to know (edit: except the perf stuff, which changes as the js engines get better). I'll second the Eloquent Javascript recommendation in another comment if you're looking for a more involved book.

[1] http://bonsaiden.github.com/JavaScript-Garden/

With the JS quirks out of the way, I would recommend building something in React and muddle through things with SO/google.

The hardest part of frontend dev for an experienced backend dev is the CSS. CSS is conceptually simple but there's a LOT of random domain knowledge you need to get layouts you want. You can bypass a lot of it by using a sass framework (e.g. foundation or bourbon) at the cost of adding a build step and another abstraction layer on top of CSS.

You should def know JS prior to learning React.

This is a pretty great book

http://eloquentjavascript.net/

Both.

Alternate your time spent learning.

Sometimes study high-level, sometimes low-level. (Low-level, tongue-in-cheek, as in "JS is the bytecode of web applications.")

I have a similar path, and honestly found the FE dev tools being the slowest thing to learn about (too many, too frequently updated/breaking things).

I think you can build a lot on top of React with minimal knowledge of js (so, yes, start), but you definitely want to go deep and understand how it works inside (so, yes, also study js).

There's no need to go deep diving on learning JavaScript.

As you're using a high level framework, you're interacting with that more than the details of JavaScript. So in essence you're learning 'React' not 'JavaScript'.

So just start building in React and the JS will come along naturally.

That's not really true. You really need to understand JS to work with React, especially since you need to combine it with other libs since it's only the view layer.