I have a huge angularjs app with many .js files.
As far I know browser should open a new connections to reach every single file. File size doesn't matter for performance but opening new connections do.
So that's why I'm looking for a some js-assembler to make single .js file.
I am not sure I should do this in case of angular app? Can anyone advice me which js assembler I should choose?
As you mentioned you have two projects one's backend is Nodejs, the other is Java, so I will address this question into this two aspects:
Just like Antoine said, you can use some building tools like gruntjs(http://www.gruntjs.com/) to write a simple task to concatenate all your js files. Or you can serve the concatenated file from backbend using tools like connect-assets(if you happen use connect web framework), more details please refer this question: Concat and minify JS files in Node.
wro4j is pretty handy, which provide pretty much everything you need to deal with assets in Java project. You can check out the details usage in their github page. Other than this, Google Closure Compiler is also a good choice.
Hope it helps, thanks.
This recipe can be found in it's original form on Stack Over Flow.