Update / clarify docs for JS library usage

See #24276

Author: sbc100

site/source/docs/porting/connecting_cpp_and_javascript/Interacting-with-code.rst

@@ -362,20 +362,18 @@ Implement a C API in JavaScript
 It is possible to implement a C API in JavaScript! This is the approach
 used in many of Emscripten's libraries, like SDL1 and OpenGL.
 
-You can use it to write your own APIs to call from C/C++. To do this
-you define the interface, decorating with ``extern`` to mark the methods
-in the API as external symbols. You then implement the symbols in
-JavaScript by simply adding their definition to `library.js`_ (by
-default). When compiling the C code, the compiler looks in the JavaScript
-libraries for relevant external symbols.
-
-By default, the implementation is added to **library.js** (and this is
-where you'll find parts of Emscripten's *libc*). You can put
-the JavaScript implementation in your own library file and add it using
-the :ref:`emcc option <emcc-js-library>` ``--js-library``. See
-`test_jslib`_ in **test/test_other.py** for a complete working
-example, including the syntax you should use inside the JavaScript library
-file.
+You can use it to write your own APIs to call from C/C++. To do this you define
+the interface, decorating with ``extern`` to mark the methods in the API as
+external symbols. You can them implement the symbols in JavaScript by simply
+adding their definition to one of the `core JS library`_ files.  Undefined 
+native symbols will be resolved by looking for them in JavaScript library files.
+
+The `core JS library`_ files are where you will find Emscripten internals. For
+example, parts of Emscripten's *libc* are implemented there. You can also put
+the JavaScript implementation in your own library file and add it using the
+:ref:`emcc option <emcc-js-library>` ``--js-library``. See `test_jslib`_ in
+**test/test_other.py** for a complete working example, including the syntax you
+should use inside the JavaScript library file.
 
 As a simple example, consider the case where you have some C code like this:
 
@@ -843,8 +841,8 @@ on Emscripten. If you would like to port existing Node-API addon to WebAssembly
 or compile the same binding code to both Node.js native addon and WebAssembly,
 you can give it a try. See `Emnapi documentation`_ for more details.
 
-.. _library.js: https://github.com/emscripten-core/emscripten/blob/main/src/library.js
-.. _test_jslib: https://github.com/emscripten-core/emscripten/blob/1.29.12/tests/test_core.py#L5043
+.. _core JS library: https://github.com/emscripten-core/emscripten/blob/main/src/lib/
+.. _test_jslib: https://github.com/emscripten-core/emscripten/blob/4.0.9/test/test_core.py#L6261
 .. _tools/system_libs.py: https://github.com/emscripten-core/emscripten/blob/main/tools/system_libs.py
 .. _library_\*.js: https://github.com/emscripten-core/emscripten/tree/main/src
 .. _test_add_function in test/test_core.py: https://github.com/emscripten-core/emscripten/blob/1.29.12/tests/test_core.py#L6237