Within directory embind-test-01, compile with
emcc --bind -o test.js test.cpp
and open up test.html within Firefox-browser. Then, within the console you see "11" and "hello".

Then, compile with
emcc --bind -o test.js test.cpp -O2
and open up test.html within Firefox-browser. Then, within the console you see "TypeError: Module.MyClass is not a constructor"

I'm using the portable SDK. "./emsdk list" gives:

The following individual tools exist:
clang-incoming-32bit
clang-incoming-64bit
clang-master-32bit
X clang-master-64bit INSTALLED
emscripten-1.29.0
emscripten-1.30.0
emscripten-incoming-32bit
emscripten-master-32bit
emscripten-incoming-64bit
X emscripten-master-64bit INSTALLED

The following Emscripten SDK versions are available:
sdk-incoming-32bit
sdk-incoming-64bit
sdk-master-32bit
X sdk-master-64bit INSTALLED

Items marked with * are activated for the current user.

Any ideas why this is happening?

Follow the build-instructions in emscripten-tests/libjpeg-01/README.txt. The resulting JavaScript will be of a size of about 4 MB, because it is built using -O0, without using the closure-compiler.

For increasing execution-speed and for reducing the size, I'd like to compile with at least -O2, better with -Oz.

So far I've found no way to this. It tried just calling emcc with -O2/-Oz instead of -O0. I additionally tried to compile libjpeg with emmake make -e CPPFLAGS=-Oz, before compiling with emcc -Oz, but that did not work either. I get exceptions like this all the time:

uncaught exception: abort(0) at jsStackTrace@file:///home/user/emscripten/jpeg-components/ipl-wrapper/emscripten-tests/libjpeg-01/src/DecodeImage.js:1:21746
stackTrace@file:///home/user/emscripten/jpeg-components/ipl-wrapper/emscripten-tests/libjpeg-01/src/DecodeImage.js:1:21929
abort@file:///home/user/emscripten/jpeg-components/ipl-wrapper/emscripten-tests/libjpeg-01/src/DecodeImage.js:15:35454
nullFunc_vii@file:///home/user/emscripten/jpeg-components/ipl-wrapper/emscripten-tests/libjpeg-01/src/DecodeImage.js:1:192767
im@file:///home/user/emscripten/jpeg-components/ipl-wrapper/emscripten-tests/libjpeg-01/src/DecodeImage.js:8:33536
he@file:///home/user/emscripten/jpeg-components/ipl-wrapper/emscripten-tests/libjpeg-01/src/DecodeImage.js:5:29651
$d@file:///home/user/emscripten/jpeg-components/ipl-wrapper/emscripten-tests/libjpeg-01/src/DecodeImage.js:5:11423
Qd@file:///home/user/emscripten/jpeg-components/ipl-wrapper/emscripten-tests/libjpeg-01/src/DecodeImage.js:5:4413
Pd@file:///home/user/emscripten/jpeg-components/ipl-wrapper/emscripten-tests/libjpeg-01/src/DecodeImage.js:5:3870
ud@file:///home/user/emscripten/jpeg-components/ipl-wrapper/emscripten-tests/libjpeg-01/src/DecodeImage.js:5:868
Id@file:///home/user/emscripten/jpeg-components/ipl-wrapper/emscripten-tests/libjpeg-01/src/DecodeImage.js:5:2636
pl@file:///home/user/emscripten/jpeg-components/ipl-wrapper/emscripten-tests/libjpeg-01/src/DecodeImage.js:8:31500
dynCall_viiii_5@file:///home/user/emscripten/jpeg-components/ipl-wrapper/emscripten-tests/libjpeg-01/src/DecodeImage.js line 1 > Function:2:12
DecodeImage$initialize@file:///home/user/emscripten/jpeg-components/ipl-wrapper/emscripten-tests/libjpeg-01/src/DecodeImage.js line 1 > Function:8:1
processJpgArray@file:///home/user/emscripten/jpeg-components/ipl-wrapper/emscripten-tests/libjpeg-01/src/loadJpgs.html:28:9
loadJpgFile/fileReader.onload@file:///home/user/emscripten/jpeg-components/ipl-wrapper/emscripten-tests/libjpeg-01/src/loadJpgs.html:99:11

"Invalid function pointer '0' called with signature 'vii'. Perhaps this is an invalid value (e.g. caused by calling a virtual method on a NULL pointer)? Or calling a function with an incorrect type, which will fail? (it is worth building your source files with -Werror (warnings are errors), as warnings can indicate undefined behavior which can cause this)" DecodeImage.js:1:2539
"This pointer might make sense in another type signature: vi: 0 viii: 0 v: 0 viiii: 0 viiiii: 0 viiiiii: 0 viiiiiii: 0 ii: 0 iii: 0 i: 0 iiii: 0 iiiii: 0 iiiiii: 0 iiiiiii: 0 " DecodeImage.js:1:2539
0 DecodeImage.js:1:2460
#0

Any ideas how to fix this?

Work within the directory webidl-test-01. First, create the glue-code:
python tools/webidl_binder.py test.idl glue

Then compile:
emcc test.cpp my_glue_wrapper.cpp --post-js glue.js -o test.js -Oz

Open test.html in Firefox-browser, you'll see "11" in the console, which is the expected output.

Then, compile with
emcc test.cpp my_glue_wrapper.cpp --post-js glue.js -o test.js -Oz --closure 1

Open test.html in Firefox-browser, you'll see "ReferenceError: Module is not defined".

I'm using the portable SDK. "./emsdk list" gives:

The following individual tools exist:
clang-incoming-32bit
clang-incoming-64bit
clang-master-32bit
X clang-master-64bit INSTALLED
emscripten-1.29.0
emscripten-1.30.0
emscripten-incoming-32bit
emscripten-master-32bit
emscripten-incoming-64bit
X emscripten-master-64bit INSTALLED

The following Emscripten SDK versions are available:
sdk-incoming-32bit
sdk-incoming-64bit
sdk-master-32bit
X sdk-master-64bit INSTALLED

Items marked with * are activated for the current user.

Well, the workflow is a bit more complicated than with embind-test-01, but at least compiler-optimization can be enabled. But unfortunately, using the closure-compiler for further reduction of size, does not work.

Any ideas why this is happening?