From 9b81e2bd56a49d6ef2c57ba73089ead09d6e6e19 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Matthias=20Fre=C3=9Fdorf?= <mafr@adorsys.de>
Date: Wed, 25 Aug 2021 20:38:34 +0200
Subject: [PATCH] Keep original function implementation when not matched

---
 src/when.js      |  5 +++--
 src/when.test.js | 25 +++++++++++++++++++++++++
 2 files changed, 28 insertions(+), 2 deletions(-)

diff --git a/src/when.js b/src/when.js
index 58435e9..e75a090 100644
--- a/src/when.js
+++ b/src/when.js
@@ -96,8 +96,9 @@ class WhenMock {
             return mockImplementation(...args)
           }
         }
-
-        return defaultImplementation ? defaultImplementation(...args) : undefined
+        return defaultImplementation ? defaultImplementation(...args)
+          : (typeof fn.__whenMock__._origMock === 'function'
+            ? fn.__whenMock__._origMock(...args) : undefined)
       })
 
       return {
diff --git a/src/when.test.js b/src/when.test.js
index 64b99ae..cf161c7 100644
--- a/src/when.test.js
+++ b/src/when.test.js
@@ -835,5 +835,30 @@ describe('When', () => {
       const returnValue = theInstance.theMethod(1)
       expect(returnValue).toBe('mock')
     })
+
+    it('keeps default function implementation when not matched', () => {
+      class TheClass {
+        fn () {
+          return 'real'
+        }
+      }
+      const instance = new TheClass()
+      const spy = jest.spyOn(instance, 'fn')
+      when(spy)
+        .calledWith(1)
+        .mockReturnValue('mock')
+      expect(instance.fn(2)).toBe('real')
+    })
+
+    it('keeps default mock implementation when not matched', () => {
+      const fn = jest.fn(() => {
+        return 'real'
+      })
+      when(fn)
+        .calledWith(1)
+        .mockReturnValue('mock')
+      expect(fn(1)).toBe('mock')
+      expect(fn(2)).toBe('real')
+    })
   })
 })