Is there any guide or examples to write plain js for blocks?
What do you mean by plain js for blocks?
Main idea is to use same selectors in JS as you use in CSS.
You may also keep JS files of each block in block's folder as well as CSS and any other techs (see https://en.bem.info/method/filesystem/).
plain js for blocks
But you may also use some libraries. E.g. here's a jQuery plugin to easily work with blocks, elements and modifiers: http://xslc.org/jquery-bem/
Or you may take a look at more powerful solution called i-bem.js: https://en.bem.info/technology/i-bem/v2/i-bem-js/
Using js- prefix is bad practice, try to avoid it. Just use the same class for css and js functionality.