In Emscripten, writing to `std::cout` in some cases fails with "TypeError: Cannot read property 'fd' of null".

helper.js

mergeInto(LibraryManager.library, {
  try_from_js: function() {
    Module.ccall('print_it', 'number', ['string'], ["From `try_from_js` it works."]);
    setTimeout(function() {
      // It could be also nodejs' readline's `on('line', …)` callback.
      Module.ccall('print_it', 'number', ['string'], ["From `setTimeout` callback it fails."]);
    }, 0);
  }
});

Makefile

TARGET = why-not-works.js

OBJS = src.bc
HLPS = helper.js

CXX = emcc

LDFLAGS = \
  -s EXPORTED_FUNCTIONS="['_print_it', '_main']" \
  --js-library helper.js

.PHONY: run clean default

$(TARGET): $(OBJS) $(HLPS)
	$(CXX) -o $@ $(OBJS) $(LDFLAGS)

%.bc: %.cpp
	$(CXX) $< -o $@

default: $(TARGET)

clean:
	- rm $(TARGET) *.bc

run: $(TARGET)
	node $(TARGET)

RESULT

$ make clean run
rm why-not-works.js *.bc
emcc src.cpp -o src.bc
emcc -o why-not-works.js src.bc -s EXPORTED_FUNCTIONS="['_print_it', '_main']" --js-library helper.js
node why-not-works.js
 * From `main` it works.
 * From `try_from_js` it works.

/Users/skale/Projekty/cannot-read-fd/why-not-works.js:7959
$67,$90,$93,$102,$147) } catch(e) { if (typeof e != "number") throw e; if (ABO
                                                                    ^
TypeError: Cannot read property 'fd' of null
    at _fileno (/Users/skale/Projekty/cannot-read-fd/why-not-works.js:5597:41)
    at _fwrite (/Users/skale/Projekty/cannot-read-fd/why-not-works.js:5603:16)
    at Array.__ZNSt3__111__stdoutbufIcE6xsputnEPKci [as 700] (/Users/skale/Projekty/cannot-read-fd/why-not-works.js:9590:9)
    at __ZNSt3__116__pad_and_outputIcNS_11char_traitsIcEEEENS_19ostreambuf_iteratorIT_T0_EES6_PKS4_S8_S8_RNS_8ios_baseES4_ (/Users/skale/Projekty/cannot-read-fd/why-not-works.js:8519:31)
    at /Users/skale/Projekty/cannot-read-fd/why-not-works.js:7959:44
    at __ZNSt3__1lsINS_11char_traitsIcEEEERNS_13basic_ostreamIcT_EES6_PKc (/Users/skale/Projekty/cannot-read-fd/why-not-works.js:7959:290)
    at _print_it (/Users/skale/Projekty/cannot-read-fd/why-not-works.js:7742:9)
    at ccallFunc (/Users/skale/Projekty/cannot-read-fd/why-not-works.js:612:24)
    at Object.ccall (/Users/skale/Projekty/cannot-read-fd/why-not-works.js:569:10)
    at null._onTimeout (/Users/skale/Projekty/cannot-read-fd/why-not-works.js:2302:16)
make: *** [run] Error 8

src.cpp

#import <iostream>

extern "C" void try_from_js();

extern "C" int print_it(const char* str) {
  std::cout << " * " << str << std::endl;
  return 0;
}

int main(int argc, char* argv[]) {
  print_it("From `main` it works.");
  try_from_js();
  return 0;
}