mockReturnValueOnce ignores calls with undefined (docs should say so)

[airandfingers]

I have a mock function that I want to return undefined in certain cases, because that's what it actually returns.

I called myMockFn.mockReturnValueOnce(undefined), and was puzzled when I got the default return value instead of undefined.

I see from the relevant code that undefined values aren't supported; they'll always be overridden by the default value.

I'm find with this—I can work around it—but IMO the docs should mention somewhere that undefined values are overridden by a mock function's default implementation.

[thymikee]

Are you willing to send a PR with the wording? 🙂

[SimenB]

That's a bug IMO. It tries to check if you provided a return value, and thinks you didn't. You should be able to mock an empty return from a function.

What about this (untested)?:

diff --git i/packages/jest-mock/src/index.js w/packages/jest-mock/src/index.js
index 9627e65e..9a7d8997 100644
--- i/packages/jest-mock/src/index.js
+++ w/packages/jest-mock/src/index.js
@@ -270,8 +270,11 @@ class ModuleMockerClass {
         // mockImplementationOnce()/mockImplementation() is called after that.
         // use the set return value.
         if (mockConfig.isReturnValueLastSet) {
-          returnValue = mockConfig.specificReturnValues.shift();
-          if (returnValue === undefined) {
+          const {specificReturnValues} = mockConfig;
+
+          if (specificReturnValues.length > 0) {
+            returnValue = specificReturnValues.shift();
+          } else {
             returnValue = mockConfig.defaultReturnValue;
           }
         }

[SimenB]

That didn't work, but sent a PR for it, #3354

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