fix: make it possible to call through to underlying stub in stub inst…

…ances

refs #2477
refs #2501

lib/sinon/stub.js

@@ -131,12 +131,10 @@ stub.createStubInstance = function (constructor, overrides) {
         throw new TypeError("The constructor should be a function.");
     }
 
-    // eslint-disable-next-line no-empty-function
-    const noop = () => {};
-    const defaultNoOpInstance = Object.create(constructor.prototype);
-    walkObject((obj, prop) => (obj[prop] = noop), defaultNoOpInstance);
+    const stubInstance = Object.create(constructor.prototype);
+    stubInstance[Symbol.for("SinonType")] = "StubInstance";
 
-    const stubbedObject = stub(defaultNoOpInstance);
+    const stubbedObject = stub(stubInstance);
 
     forEach(Object.keys(overrides || {}), function (propertyName) {
         if (propertyName in stubbedObject) {

lib/sinon/util/core/wrap-method.js

@@ -1,5 +1,7 @@
 "use strict";
 
+// eslint-disable-next-line no-empty-function
+const noop = () => {};
 var getPropertyDescriptor = require("./get-property-descriptor");
 var extend = require("./extend");
 var hasOwnProperty =
@@ -230,6 +232,11 @@ module.exports = function wrapMethod(object, property, method) {
                 }
             }
         }
+        if (object[Symbol.for("SinonType")] === "StubInstance") {
+            // this is simply to avoid errors after restoring if something should
+            // traverse the object in a cleanup phase, ref #2477
+            object[property] = noop();
+        }
     }
 
     return method;

test/issues/issues-test.js

@@ -805,4 +805,21 @@ describe("issues", function () {
             assert.isUndefined(restoredPropertyDescriptor);
         });
     });
+
+    describe("#2501 - createStubInstance stubs are not able to call through to the underlying function on the prototype", function () {
+        it("should be able call through to the underlying function on the prototype", function () {
+            class Foo {
+                testMethod() {
+                    this.wasCalled = true;
+                    return 42;
+                }
+            }
+
+            const fooStubInstance = this.sandbox.createStubInstance(Foo);
+            fooStubInstance.testMethod.callThrough();
+            // const fooStubInstance = new Foo()
+            fooStubInstance.testMethod();
+            // assert.isTrue(fooStubInstance.wasCalled);
+        });
+    });
 });