title: Unknown JavaScript syntax before for...in
body: <p>I am reading a book about single-page applications and at some point there is this for...in loop:</p>
  
  <pre><code>KEYVAL:
  for(key_name in arg_map){
      if(arg_map.hasOwnProperty(key_name)){
          if(key_name.indexOf('_') === 0) continue KEYVAL;
  
          anchor_map_revise[key_name] = arg_map[key_name];
  
          key_name_dep = '_' + key_name;
          if(arg_map[key_name_dep]){
              anchor_map_revise[key_name_dep] = arg_map[key_name_dep];
          }
          else{
              delete anchor_map_revise[key_name_dep];
              delete anchor_map_revise['_s' + key_name_dep];
          }
      }
  }
  </code></pre>
  
  <p>What really caught my eye was the <code>KEYVAL</code> word right before the loop. Is it a variable? What does it represent? What is it for? What does this syntax mean? The word only appears in the two places in the included code and never again in the whole example.</p>
  
  <p>I've been searching a lot trying to figure this out, but so far I have not been able to find any information. Could someone please help me out?</p>
  
  <p>Thank you.</p>