A basic example of '--js-library' external JavaScript library usage with Emscripten?

Hi,
Before I go on, I'm a newbie to both Emscripten and GCC (I'm a Java dev).

I think I've already grasped the 'EXPORTED_FUNCTIONS' flag of Emscripten/emcc compiler as highlighting the C/C++ functions we want to hang on to/access after compilation to JavaScript, however, the inclusion of 3rd party Javascript libraries, to emcc's compilation phase (via the '--js-library' flag) is confusing me, a bit.

How are these extra JavaScript files being used (or not) from the yet-to-be-compiled C/C++ target code? Can you explain or give a simple example?

Thanks