Fields without initalizer don't have the initializer function

(Review by @littledan)

packages/babel-plugin-proposal-decorators/src/transformer.js

@@ -45,7 +45,7 @@ function getKey(node) {
 }
 
 function getSingleElementDefinition(path, superRef, classRef, file) {
-  const { node } = path;
+  const { node, scope } = path;
   const isMethod = path.isClassMethod();
 
   if (path.isPrivate()) {
@@ -61,10 +61,10 @@ function getSingleElementDefinition(path, superRef, classRef, file) {
       methodPath: path,
       methodNode: node,
       objectRef: classRef,
-      superRef: superRef,
       isStatic: node.static,
-      scope: path.scope,
-      file: file,
+      superRef,
+      scope,
+      file,
     },
     true,
   ).replace();
@@ -76,7 +76,9 @@ function getSingleElementDefinition(path, superRef, classRef, file) {
     prop("key", getKey(node)),
     isMethod
       ? value(node.body, node.params)
-      : value(template.ast`{ return ${node.value} }`),
+      : node.value
+        ? value(template.ast`{ return ${node.value} }`)
+        : prop("value", scope.buildUndefinedNode()),
   ].filter(Boolean);
 
   return t.objectExpression(properties);

packages/babel-plugin-proposal-decorators/test/fixtures/element-descriptors/original-own-field-without-initiailzer/exec.js

@@ -0,0 +1,18 @@
+var el = null;
+
+class A {
+  @(_ => el = _)
+  foo;
+}
+
+expect(el).toEqual(Object.defineProperty({
+  kind: "field",
+  key: "foo",
+  placement: "own",
+  descriptor: {
+    enumerable: false,
+    configurable: true,
+    writable: true,
+  },
+  initializer: undefined,
+}, Symbol.toStringTag, { value: "Descriptor" }));