title: Find the key existence in JSON object
body: <p>I have a JSON string</p>
  
  <pre><code>String str = '{ "name":"John", "age":31, "city":"New York" }'
  </code></pre>
  
  <p>In the above string, <strong>city</strong> is an optional key and may not be present in the JSON data.</p>
  
  <p>I decoded the json string</p>
  
  <pre><code>var jsonResponse = json.decode(str);
  </code></pre>
  
  <p>I want to get to know the existence of <strong>city</strong> key the <strong>jsonResponse</strong> object.</p>