title: Expect assertions type error -> expect(...).toExist is not a function
body: <p>I'm testing a NodeJS app. I encountered this error when I ran the tests. The test script is below:</p>
  <pre class="lang-js prettyprint-override"><code>.expect((res) =&gt; {
      expect(res.headers['x-auth']).toExist();
      expect(res.body._id).toExist();
      expect(res.body.email).toBe(email);
  })
  </code></pre>
  <p>The error showed:</p>
  <pre class="lang-none prettyprint-override"><code>TypeError: expect(...).toExist is not a function
  </code></pre>
  <p>How can I resolve this issue?</p>