title: What does this immutable property in options object of express.static used for?
body: <p>I am a newbie to Node and Express, trying to explore things. I was going through the Express' documentation at this link: <a href="https://expressjs.com/en/4x/api.html#express.static" rel="nofollow noreferrer">https://expressjs.com/en/4x/api.html#express.static</a><br>
  Here, The description of the <code>immutable</code> property passed in options says:</p>
  
  <blockquote>
    <p>Enable or disable the immutable directive in the Cache-Control response header. If enabled, the maxAge option should also be specified to enable caching. The immutable directive will prevent supported clients from making conditional requests during the life of the maxAge option to check if the file has changed.</p>
  </blockquote>
  
  <p>I am unable to understand this. I've understood how <code>maxAge</code> is used but not able to get this immutable.
  <br>
  When and How is this property used?<br><br>
  Thanks in advance for any help you are able to provide</p>