Simplify _initProto injection for existing super()

packages/babel-helpers/src/helpers/applyDecs.js

@@ -518,9 +518,12 @@ function pushInitializers(ret, initializers) {
       for (var i = 0; i < initializers.length; i++) {
         initializers[i].call(instance, instance);
       }
+      return instance;
     });
   } else {
-    ret.push(function () {});
+    ret.push(function (instance) {
+      return instance;
+    });
   }
 }
 

packages/babel-plugin-proposal-decorators/src/transformer-2021-12.ts

@@ -796,11 +796,7 @@ function transformClass(
               if (!path.get("callee").isSuper()) return;
 
               path.replaceWith(
-                t.sequenceExpression([
-                  path.node,
-                  t.cloneNode(protoInitCall),
-                  t.thisExpression(),
-                ]),
+                t.callExpression(t.cloneNode(protoInitLocal), [path.node]),
               );
 
               path.skip();

packages/babel-plugin-proposal-decorators/test/fixtures/2021-12-misc/initProto-existing-derived-constructor-multiple-super/output.js

@@ -7,9 +7,9 @@ class A extends B {
 
   constructor() {
     if (Math.random() > 0.5) {
-      super(true), _initProto(this), this;
+      _initProto(super(true));
     } else {
-      super(false), _initProto(this), this;
+      _initProto(super(false));
     }
   }
 
@@ -24,7 +24,7 @@ class C extends B {
 
   constructor() {
     try {
-      super((super(), _initProto2(this), this), null.x), _initProto2(this), this;
+      _initProto2(super(_initProto2(super()), null.x));
     } catch {}
   }
 

packages/babel-plugin-proposal-decorators/test/fixtures/2021-12-misc/initProto-existing-derived-constructor/exec.js

@@ -0,0 +1,28 @@
+let self, a, initCalled;
+
+function deco(_, context) {
+  context.addInitializer(() => {
+    initCalled = true;
+  })
+}
+
+class B {
+  constructor(s) {
+    a = s;
+  }
+}
+
+class A extends B {
+  constructor() {
+    let a = 2;
+    self = super(a);
+  }
+
+  @deco
+  method() {}
+}
+
+let instance = new A();
+expect(self).toBe(instance);
+expect(a).toBe(2);
+expect(initCalled).toBe(true);

packages/babel-plugin-proposal-decorators/test/fixtures/2021-12-misc/initProto-existing-derived-constructor/output.js

@@ -7,7 +7,9 @@ class A extends B {
 
   constructor() {
     let a = 2;
-    super(a), _initProto(this), this;
+
+    _initProto(super(a));
+
     foo();
   }