title: Is jest.mock() equivalent to sinon.stub()
body: <p>In all examples testing of Firebase SDK for Cloud Functions Quickstart, Mocha/Chai and Sinon are used... Trying to use Jest instead , I wonder what is the correct equivalent of sinon.stub() ?</p>
  
  <p>I tried the following (only for testing jest.mock() acceptance...</p>
  
  <pre><code>const sinon = require('sinon');
  
  const jest = require('jest');
  
  describe('Cloud Functions', () =&gt; {
  
      before(() =&gt; {
        let myFunctions, adminInitStub;
        let adminInitMock;
  
        adminInitStub = sinon.stub(admin, 'initializeApp');
        adminInitMock = jest.mock(admin, 'initializeApp');
  
        myFunctions = require('../index');
        myFunctions = require('../index');
  </code></pre>
  
  <p>but I get an error :</p>
  
  <p>1 failing</p>
  
  <p>1) Cloud Functions
         "before all" hook:
       TypeError: jest.mock is not a function</p>
  
  <p>I wrong somewhere ... but I cannot get it 
  thanks for feedback</p>