diff --git a/magic/Makefile b/magic/Makefile
index 764b33c2..a7dc9a7f 100644
--- a/magic/Makefile
+++ b/magic/Makefile
@@ -60,9 +60,9 @@ ifneq (${TCL_LIB_DIR},)
 # the stub-bootstrap archive (-ltclstub). Both are required: magic's objects
 # use USE_TCL_STUBS macros (resolved by tclStubsPtr from libtclstub.a), and
 # tclStubsPtr itself must point into the real TCL implementation (libtcl9.x.a).
-	${CC} ${CFLAGS} ${CPPFLAGS} ${DFLAGS} lib${MODULE}.o ${EXTRA_LIBS} -o magic.js ${LIBS} ${LIB_SPECS_NOSTUB} -L${TCL_LIB_DIR} -ltclstub
+	${CC} ${CFLAGS} ${CPPFLAGS} ${DFLAGS} ${LDFLAGS} lib${MODULE}.o ${EXTRA_LIBS} -o magic.js ${LIBS} ${LIB_SPECS_NOSTUB} -L${TCL_LIB_DIR} -ltclstub
 else
-	${CC} ${CFLAGS} ${CPPFLAGS} ${DFLAGS} lib${MODULE}.o ${EXTRA_LIBS} -o magic.js ${LIBS}
+	${CC} ${CFLAGS} ${CPPFLAGS} ${DFLAGS} ${LDFLAGS} lib${MODULE}.o ${EXTRA_LIBS} -o magic.js ${LIBS}
 endif
 endif
 
diff --git a/npm/build.sh b/npm/build.sh
index f730e103..20923d7f 100755
--- a/npm/build.sh
+++ b/npm/build.sh
@@ -95,8 +95,21 @@ sed_strip_cr() {
 
 if [ $OPT_RELEASE -eq 1 ]; then
   EXTRA_CFLAGS="-O2"
+  EXTRA_LDFLAGS=""
 else
   EXTRA_CFLAGS="-g"
+  EXTRA_LDFLAGS="-gsource-map"
+
+  # Exists to document some options maybe placement sensitive on linker command
+  # line and need to appear first.
+  # See toolchains/emscripten/defs.mak this is just picked up from the environment during make
+  #export TOP_FIRST_LIBS_WASM="-g3 -fsanitize=address"
+
+  # Exists to document maybe you need both these to get the intended effect
+  # but expect the performance hit / larger codegen at runtime when you do.
+  # See toolchains/emscripten/defs.mak this is just picked up from the environment during make
+  #EXTRA_CONFIGURE_ARGS="--enable-assertions"
+  #export TOP_EXTRA_LIBS_WASM="-sASSERTIONS=1" # emscripten default value: 1
 fi
 
 # --- TCL fork: clone and prebuild (TCL variant only) ------------------------
@@ -163,6 +176,7 @@ build_variant() {
   if [ "$variant" = "tcl" ]; then
     ensure_tcl_built
     CFLAGS="--std=c17 -D_DEFAULT_SOURCE=1 -DEMSCRIPTEN=1 ${EXTRA_CFLAGS}" \
+    LDFLAGS="${EXTRA_LDFLAGS}" \
       emconfigure ./configure \
         --without-cairo --without-opengl --without-x --without-tk \
         --with-tcl="$TCL_WASM_PREFIX/lib" \
@@ -170,19 +184,27 @@ build_variant() {
         --with-tcllibs="$TCL_WASM_PREFIX/lib" \
         --disable-readline --disable-compression \
         --host=asmjs-unknown-emscripten \
-        --target=asmjs-unknown-emscripten
+        --target=asmjs-unknown-emscripten \
+        ${EXTRA_CONFIGURE_ARGS:-}
   else
     CFLAGS="--std=c17 -D_DEFAULT_SOURCE=1 -DEMSCRIPTEN=1 ${EXTRA_CFLAGS}" \
+    LDFLAGS="${EXTRA_LDFLAGS}" \
       emconfigure ./configure \
         --without-cairo --without-opengl --without-x \
         --without-tk --without-tcl \
         --disable-readline --disable-compression \
         --host=asmjs-unknown-emscripten \
-        --target=asmjs-unknown-emscripten
+        --target=asmjs-unknown-emscripten \
+        ${EXTRA_CONFIGURE_ARGS:-}
   fi
 
   cat toolchains/emscripten/defs.mak >> defs.mak
 
+  #
+  echo "===== defs.mak ====="
+  cat defs.mak
+  echo "===== defs.mak ====="
+
   emmake make depend
   emmake make -j"$(ncpu)" modules libs
   emmake make techs
@@ -191,6 +213,7 @@ build_variant() {
   mkdir -p "$out_dir"
   cp magic/magic.js   "$out_dir/"
   cp magic/magic.wasm "$out_dir/"
+  test -f magic/magic.wasm.map && cp magic/magic.wasm.map "$out_dir/"
   echo "Copied magic.js + magic.wasm into npm/$variant/"
 }
 
diff --git a/npm/package.json b/npm/package.json
index 8667a4d5..5564227a 100644
--- a/npm/package.json
+++ b/npm/package.json
@@ -17,8 +17,10 @@
     "notcl.js",
     "tcl/magic.js",
     "tcl/magic.wasm",
+    "tcl/magic.wasm.map",
     "notcl/magic.js",
     "notcl/magic.wasm",
+    "notcl/magic.wasm.map",
     "examples/",
     "LICENSE",
     "README.md"
diff --git a/toolchains/emscripten/defs.mak b/toolchains/emscripten/defs.mak
index c5f21ea5..c1137bea 100644
--- a/toolchains/emscripten/defs.mak
+++ b/toolchains/emscripten/defs.mak
@@ -4,6 +4,7 @@
 #
 #   source <emsdk>/emsdk_env.sh
 #   CFLAGS="--std=c17 -D_DEFAULT_SOURCE=1 -DEMSCRIPTEN=1 -g" \
+#   LDFLAGS="-gsource-maps" \
 #     emconfigure ./configure --without-cairo --without-opengl --without-x \
 #                             --without-tk   --without-tcl  \
 #                             --disable-readline --disable-compression \
@@ -30,6 +31,7 @@ MAKE_WASM = 1
 # consumers passing locateFile/arguments/etc. keep working) and re-add
 # wasmBinary. Keep this in sync with emscripten's default if it grows.
 TOP_EXTRA_LIBS += \
+    ${TOP_FIRST_LIBS_WASM} \
     -sWASM=1 \
     -sMODULARIZE=1 \
     -sEXPORT_ES6=1 \
@@ -40,9 +42,9 @@ TOP_EXTRA_LIBS += \
     -sALLOW_MEMORY_GROWTH=1 \
     -sINITIAL_MEMORY=67108864 \
     -Wl,-z,stack-size=10485760 \
-    -sASSERTIONS=1 \
     -sENVIRONMENT=node,web,worker \
     -sFORCE_FILESYSTEM=1 \
+    ${TOP_EXTRA_LIBS_WASM} \
     --embed-file ../scmos@/magic/sys/current \
     --embed-file ../windows/windows7.glyphs@/magic/sys/windows7.glyphs \
     --embed-file ../windows/windows7.glyphs@/magic/sys/bw.glyphs