Warn on mixing ESM and commonJS

js/compiler/compiler.go

@@ -251,7 +251,15 @@ func (c *Compiler) compileImpl(
 			return nil, code, err
 		}
 		// the compatibility mode "decreases" here as we shouldn't transform twice
-		return c.compileImpl(code, filename, wrap, lib.CompatibilityModeBase, state.srcMap)
+		var prg *sobek.Program
+		prg, code, err = c.compileImpl(code, filename, wrap, lib.CompatibilityModeBase, state.srcMap)
+		if err == nil && strings.Contains(src, "module.exports") {
+			c.logger.Warningf(
+				"While compiling %q it was noticed that it mixes import/export syntax (ESM) and commonJS module.exports. "+
+					"This isn't standard behaviour and will soon not work. Please use one or the other.",
+				filename)
+		}
+		return prg, code, err
 	}
 
 	if compatibilityMode == lib.CompatibilityModeExperimentalEnhanced {

js/compiler/compiler_test.go

@@ -241,3 +241,26 @@ func TestMinimalSourceMap(t *testing.T) {
 	require.NoError(t, err)
 	require.Empty(t, hook.Drain())
 }
+
+func TestMixingImportExport(t *testing.T) {
+	t.Parallel()
+	logger := logrus.New()
+	logger.SetLevel(logrus.DebugLevel)
+	logger.Out = io.Discard
+	hook := testutils.NewLogHook(logrus.InfoLevel, logrus.WarnLevel)
+	logger.AddHook(hook)
+
+	compiler := New(logger)
+	compiler.Options = Options{
+		CompatibilityMode: lib.CompatibilityModeExtended,
+		Strict:            true,
+	}
+	_, _, err := compiler.Compile("export let s = 5;\nmodule.exports = 'something';", "somefile", false)
+	require.NoError(t, err)
+	entries := hook.Drain()
+	require.Len(t, entries, 1)
+	msg, err := entries[0].String() // we need this in order to get the field error
+	require.NoError(t, err)
+
+	require.Contains(t, msg, `it was noticed that it mixes`)
+}