Make sure source type is module when parsing .mjs (#5700)

* feat: force source type module for mjs extension

* style: fix lint

packages/babel-core/src/config/option-manager.js

@@ -195,6 +195,10 @@ class OptionManager {
 
     const basenameRelative = path.basename(opts.filenameRelative);
 
+    if (path.extname(opts.filenameRelative) === ".mjs") {
+      opts.sourceType = "module";
+    }
+
     defaults(opts, {
       sourceFileName: basenameRelative,
       sourceMapTarget: basenameRelative,

packages/babel-core/test/option-manager.js

@@ -44,6 +44,17 @@ describe("option-manager", () => {
     });
   });
 
+  describe("source type", function() {
+    it("should set module for .mjs extension", () => {
+      const config = manageOptions({
+        sourceType: "program",
+        filename: "foo.mjs",
+      });
+
+      assert.equal(config.options.sourceType, "module");
+    });
+  });
+
   describe("presets", function() {
     function presetTest(name) {
       it(name, function() {