How to load javascript in iframe as a part of header and us it for child frames

Loading javascript in iframe like gmail loading javascript in hidden iframe(js_frame) and make it context sensitive.

From stackoverflow

• To access the JavaScript of a parent frame you simply use the parent keyword. For example:

  child.htm:

  <script>
    var childVariable = 'child';
  
    function showParentVariable() {
      alert('from child: '+parent.parentVariable);
    }
  
    window.onload = function() {
      if (!!parent.child && parent.child===window) {
        parent.showChildVariable();
      }
    }
  </script>
  

  parent.htm:

  <iframe src="child.htm"></iframe>
  <script>
    var parentVariable = 'parent';
    var child = window.frames[0];
  
    function showChildVariable() {
      alert('from parent: '+child.childVariable);
    }
  
    window.onload = function() {
      child.showParentVariable();
    }
  </script>
  

  Here is a live demo: Child Parent

  I don't know what you mean by "context sensitive" though.