From 911c498dc14dc4034ba019526bf58f8b24d77da0 Mon Sep 17 00:00:00 2001
From: Dean Sofer <ProLoser@users.noreply.github.com>
Date: Thu, 7 Dec 2017 05:10:25 -0800
Subject: [PATCH] Spy passes through calling with `new` (#1626)

This changes the spy behavior so that if the spy is `calledWithNew()` then
the original function will also get called with `new`.

This was throwing an error if you tried spying on an ES6 class object.

fixes #1265
---
 lib/sinon/spy.js | 16 ++++++++++++----
 test/spy-test.js | 10 ++++++++++
 2 files changed, 22 insertions(+), 4 deletions(-)

diff --git a/lib/sinon/spy.js b/lib/sinon/spy.js
index 9a44a33be..29c612170 100644
--- a/lib/sinon/spy.js
+++ b/lib/sinon/spy.js
@@ -17,6 +17,7 @@ var push = Array.prototype.push;
 var slice = Array.prototype.slice;
 var filter = Array.prototype.filter;
 var ErrorConstructor = Error.prototype.constructor;
+var bind = Function.prototype.bind;
 
 var callId = 0;
 
@@ -190,11 +191,18 @@ var spyApi = {
         try {
             this.invoking = true;
 
-            returnValue = (this.func || func).apply(thisValue, args);
-
             var thisCall = this.getCall(this.callCount - 1);
-            if (thisCall.calledWithNew() && typeof returnValue !== "object") {
-                returnValue = thisValue;
+
+            if (thisCall.calledWithNew()) {
+                // Call through with `new`
+                // eslint-disable-next-line new-parens
+                returnValue = new (bind.apply(this.func || func, [thisValue].concat(args)))();
+
+                if (typeof returnValue !== "object") {
+                    returnValue = thisValue;
+                }
+            } else {
+                returnValue = (this.func || func).apply(thisValue, args);
             }
         } catch (e) {
             exception = e;
diff --git a/test/spy-test.js b/test/spy-test.js
index 3d6e41a36..dc68d6d7f 100644
--- a/test/spy-test.js
+++ b/test/spy-test.js
@@ -439,6 +439,16 @@ describe("spy", function () {
             assert(called);
         });
 
+        it("passes 'new' to underlying function", function () {
+            function TestClass() {}
+
+            var SpyClass = createSpy.create(TestClass);
+
+            var instance = new SpyClass();
+
+            assert(instance instanceof TestClass);
+        });
+
         it("passs arguments to function", function () {
             var actualArgs;