title: Checking an element is disabled using Puppeteer
body: <p>I have a button that has an initial state of <code>disabled</code> - </p>
  
  <pre><code>&lt;button type = "submit" class="ant-btn ant-btn-primary ant-btn-lg" disabled&gt;
  </code></pre>
  
  <p>The <code>disabled</code> attribute is not present once the conditions are met - so the HTML becomes </p>
  
  <pre><code>&lt;button type = "submit" class="ant-btn ant-btn-primary ant-btn-lg"&gt;
  </code></pre>
  
  <p>I want to check that the button has attribute <code>disabled</code>, however since the attribute has no value in it, I am not able to find to a way to do so. </p>
  
  <p>For example, if the <code>disabled</code> attribute had something like this </p>
  
  <pre><code>&lt;button type = "submit" class="ant-btn ant-btn-primary ant-btn-lg" disabled = "disabled"&gt;
  </code></pre>
  
  <p>then I can do something like this</p>
  
  <pre><code>let button = await page.$('button');
  let valueHandle = await input.getProperty('disabled');
  assert.equal(await valueHandle.jsonValue(), 'disabled');
  </code></pre>
  
  <p>but since there is no value for the attribute, how to proceed in this case? </p>