Removes error when mocking mongoose library root object

CHANGELOG.md

@@ -32,6 +32,7 @@
 - `[jest-jasmine2]`: Throw explicit error when errors happen after test is considered complete ([#8005](https://github.com/facebook/jest/pull/8005))
 - `[jest-circus]`: Throw explicit error when errors happen after test is considered complete ([#8005](https://github.com/facebook/jest/pull/8005))
 - `[expect]` Remove duck typing and obsolete browser support code when comparing DOM nodes and use DOM-Level-3 API instead ([#7995](https://github.com/facebook/jest/pull/7995))
+- `[jest-mock]` Adds a type check to prototype to allow mocks of mongoose library without error. ([#8040](https://github.com/facebook/jest/pull/8040))
 
 ### Chore & Maintenance
 

packages/jest-mock/src/__tests__/index.test.ts

@@ -880,6 +880,14 @@ describe('moduleMocker', () => {
         expect(fn1()).toEqual('abcd');
         expect(fn2()).toEqual('abcde');
       });
+
+      it('handles a property called `prototype`', () => {
+        const mock = moduleMocker.generateFromMetadata(
+          moduleMocker.getMetadata({prototype: 1}),
+        );
+
+        expect(mock.prototype).toBe(1);
+      });
     });
   });
 

packages/jest-mock/src/index.ts

@@ -844,7 +844,8 @@ class ModuleMockerClass {
     if (
       metadata.type !== 'undefined' &&
       metadata.type !== 'null' &&
-      mock.prototype
+      mock.prototype &&
+      typeof mock.prototype === 'object'
     ) {
       mock.prototype.constructor = mock;
     }