diff --git a/emcc b/emcc
index 7e38b8964..7059c73b6 100755
--- a/emcc
+++ b/emcc
@@ -187,9 +187,13 @@ CMAKE_CONFIG = 'CMakeFiles/cmTryCompileExec.dir' in ' '.join(sys.argv)# or 'CMak
 if CONFIGURE_CONFIG or CMAKE_CONFIG:
   debug_configure = 0 # XXX use this to debug configure stuff. ./configure's generally hide our normal output including stderr so we write to a file
 
-  # whether we fake configure tests using clang - the local, native compiler - or not. if not we generate JS and use node with a shebang
-  # neither approach is perfect, you can try both, but may need to edit configure scripts in some cases
-  # by default we configure in js, which can break on local filesystem access, etc., but is otherwise accurate
+  # Whether we fake configure tests using clang - the local, native compiler - or not. if not we generate JS and use node with a shebang
+  # Beither approach is perfect, you can try both, but may need to edit configure scripts in some cases
+  # By default we configure in js, which can break on local filesystem access, etc., but is otherwise accurate so we
+  # disable this if we think we have to. A value of '2' here will force JS checks in all cases. In summary:
+  # 0 - use native compilation for configure checks
+  # 1 - use js when we think it will work
+  # 2 - always use js for configure checks
   use_js = int(os.environ.get('EMCONFIGURE_JS') or 1)
 
   if debug_configure:
@@ -199,7 +203,7 @@ if CONFIGURE_CONFIG or CMAKE_CONFIG:
 
   src = None
   for arg in sys.argv:
-    if arg.endswith('.c'):
+    if arg.endswith(SOURCE_ENDINGS):
       try:
         src = open(arg).read()
         if debug_configure: open(tempout, 'a').write('============= ' + arg + '\n' + src + '\n=============\n\n')
@@ -207,13 +211,13 @@ if CONFIGURE_CONFIG or CMAKE_CONFIG:
         pass
     elif arg.endswith('.s'):
       if debug_configure: open(tempout, 'a').write('(compiling .s assembly, must use clang\n')
-      use_js = 0
+      if use_js == 1: use_js = 0
     elif arg == '-E':
-      use_js = 0
+      if use_js == 1: use_js = 0
 
   if src:
     if 'fopen' in src and '"w"' in src:
-      use_js = 0 # we cannot write to files from js!
+      if use_js == 1: use_js = 0 # we cannot write to files from js!
       if debug_configure: open(tempout, 'a').write('Forcing clang since uses fopen to write\n')
 
   compiler = os.environ.get('CONFIGURE_CC') or (shared.CLANG if not use_js else shared.EMCC) # if CONFIGURE_CC is defined, use that. let's you use local gcc etc. if you need that
diff --git a/tests/test_other.py b/tests/test_other.py
index 8d883dfb4..c75cd56e3 100644
--- a/tests/test_other.py
+++ b/tests/test_other.py
@@ -4647,19 +4647,27 @@ int main(void) {
 
   def test_emconfigure_js_o(self):
     # issue 2994
-    for i in [0, 1]:
-      print i
-      try:
-        os.environ['EMCONFIGURE_JS'] = str(i)
-        self.clear()
-        Popen([PYTHON, path_from_root('emconfigure'), PYTHON, EMCC, '-c', '-o', 'a.o', path_from_root('tests', 'hello_world.c')]).communicate()
-        Popen([PYTHON, EMCC, 'a.o']).communicate()
-        if i == 0:
-          assert not os.path.exists('a.out.js') # native .o, not bitcode!
-        else:
-          assert 'hello, world!' in run_js(self.in_dir('a.out.js'))
-      finally:
-        del os.environ['EMCONFIGURE_JS']
+    for i in [0, 1, 2]:
+      for f in ['hello_world.c', 'files.cpp']:
+        print i, f
+        try:
+          os.environ['EMCONFIGURE_JS'] = str(i)
+          self.clear()
+          Popen([PYTHON, path_from_root('emconfigure'), PYTHON, EMCC, '-c', '-o', 'a.o', path_from_root('tests', f)]).communicate()
+          Popen([PYTHON, EMCC, 'a.o']).communicate()
+          if f == 'hello_world.c':
+            if i == 0:
+              assert not os.path.exists('a.out.js') # native .o, not bitcode!
+            else:
+              assert 'hello, world!' in run_js(self.in_dir('a.out.js'))
+          else:
+            # file access, need 2 to force js
+            if i == 0 or i == 1:
+              assert not os.path.exists('a.out.js') # native .o, not bitcode!
+            else:
+              assert os.path.exists('a.out.js')
+        finally:
+          del os.environ['EMCONFIGURE_JS']
 
   def test_emcc_c_multi(self):
     def test(args, llvm_opts=None):