title: named variable filter syntax in redux
body: <p>I have just finished learning javascript and typescript (I know redux is not typescript), but have been thouroughly confused by the syntax for redux filters:</p>
  
  <pre><code>function visibilityFilter(state = 'SHOW_ALL', action) {
     ...
  }
  </code></pre>
  
  <p>I've seem default variables in javascript, but only when the default variable is AFTER the positional variable. What is going on here? Can someone point me to documentation that explains this syntax?</p>