Fix #678: Add WASM exception support to Emscripten build (#689)

ChaiScript relies heavily on C++ exceptions for error propagation, but
the Emscripten build was missing the -fwasm-exceptions flag. Without it,
any C++ exception in the WASM module causes an abort instead of being
catchable by JavaScript. Added -fwasm-exceptions as both a compile and
link option, and added a regression test verifying exception propagation
through the eval wrapper functions.

Co-authored-by: leftibot <leftibot@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

CMakeLists.txt

@@ -440,6 +440,10 @@ if(BUILD_TESTING)
     target_link_libraries(emscripten_eval_test ${LIBS})
     add_test(NAME Emscripten_Eval_Test COMMAND emscripten_eval_test)
 
+    add_executable(emscripten_exception_test unittests/emscripten_exception_test.cpp)
+    target_link_libraries(emscripten_exception_test ${LIBS})
+    add_test(NAME Emscripten_Exception_Test COMMAND emscripten_exception_test)
+
     add_executable(threading_config_test unittests/threading_config_test.cpp)
     target_link_libraries(threading_config_test ${LIBS})
     add_test(NAME Threading_Config_Test COMMAND threading_config_test)

emscripten/CMakeLists.txt

@@ -17,9 +17,14 @@ add_definitions(-DCHAISCRIPT_NO_THREADS -DCHAISCRIPT_NO_DYNLOAD)
 add_executable(chaiscript chaiscript_em.cpp)
 target_include_directories(chaiscript PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/../include)
 
+# Enable WASM exception handling — ChaiScript relies on C++ exceptions for
+# error propagation; without this flag exceptions cause an abort in WASM.
+target_compile_options(chaiscript PRIVATE -fwasm-exceptions)
+
 # Emscripten link flags: enable embind, allow memory growth, export as ES module-compatible
 target_link_options(chaiscript PRIVATE
   --bind
+  -fwasm-exceptions
   -sALLOW_MEMORY_GROWTH=1
   -sEXPORT_ES6=0
   -sMODULARIZE=0

unittests/emscripten_exception_test.cpp

@@ -0,0 +1,72 @@
+// Test that validates exception propagation through the Emscripten eval wrapper.
+// Without proper exception support flags (-fwasm-exceptions) in the WASM build,
+// C++ exceptions would cause an abort instead of being catchable.
+
+#ifndef CHAISCRIPT_NO_THREADS
+#define CHAISCRIPT_NO_THREADS
+#endif
+
+#ifndef CHAISCRIPT_NO_DYNLOAD
+#define CHAISCRIPT_NO_DYNLOAD
+#endif
+
+#include <chaiscript/chaiscript.hpp>
+#include "../emscripten/chaiscript_eval.hpp"
+#include <cassert>
+#include <iostream>
+#include <stdexcept>
+#include <string>
+
+int main() {
+  // Verify that ChaiScript evaluation errors propagate as exceptions
+  // through the eval wrapper functions. In WASM builds without exception
+  // support, these would abort instead of throwing.
+
+  bool caught = false;
+
+  // Test 1: eval with undefined variable should throw
+  caught = false;
+  try {
+    chaiscript_eval("this_variable_does_not_exist");
+  } catch (const chaiscript::exception::eval_error &) {
+    caught = true;
+  }
+  assert(caught && "eval of undefined variable must throw eval_error");
+
+  // Test 2: evalString with a type mismatch should throw
+  caught = false;
+  try {
+    chaiscript_eval_string("1 + 2");
+  } catch (const chaiscript::exception::bad_boxed_cast &) {
+    caught = true;
+  }
+  assert(caught && "evalString with non-string result must throw bad_boxed_cast");
+
+  // Test 3: evalInt with invalid syntax should throw
+  caught = false;
+  try {
+    chaiscript_eval_int("def {}");
+  } catch (const chaiscript::exception::eval_error &) {
+    caught = true;
+  }
+  assert(caught && "evalInt with syntax error must throw eval_error");
+
+  // Test 4: eval with throw statement should propagate exception
+  caught = false;
+  try {
+    chaiscript_eval("throw(\"user exception\")");
+  } catch (const chaiscript::Boxed_Value &) {
+    caught = true;
+  } catch (...) {
+    caught = true;
+  }
+  assert(caught && "ChaiScript throw must propagate as an exception");
+
+  // Test 5: Verify normal operation still works after caught exceptions
+  chaiscript_eval("var post_exception_test = 100");
+  const int result = chaiscript_eval_int("post_exception_test");
+  assert(result == 100 && "normal eval must work after caught exceptions");
+
+  std::cout << "All emscripten exception tests passed.\n";
+  return 0;
+}