title: How to add key/value pair in Javascript object at the start
body: <p>I have following object:</p>
  
  <pre><code>var obj = {"last_name": "john", "age": 23, "city": "London"}
  </code></pre>
  
  <p>I want to add a new element of first_name at the start of this object to look like this:</p>
  
  <pre><code>{"first_name": "Samuel", "last_name": "john", "age": 23, "city": "London"}
  </code></pre>
  
  <p>I know I can do <code>obj.first_name = "Samuel"</code> but it adds the new element at the end of the object. How can I add it at the start?</p>