How to stop event propagation with inline onclick attribute?

Use event.stopPropagation().

<span onclick="event.stopPropagation(); alert('you clicked inside the header');">something inside the header</span>

For IE: window.event.cancelBubble = true

<span onclick="window.event.cancelBubble = true; alert('you clicked inside the header');">something inside the header</span>

There are two ways to get the event object from inside a function:

1. The first argument, in a W3C-compliant browser (Chrome, Firefox, Safari, IE9+)
2. The window.event object in Internet Explorer (<=8)

If you need to support legacy browsers that don't follow the W3C recommendations, generally inside a function you would use something like the following:

function(e) {
  var event = e || window.event;
  [...];
}

which would check first one, and then the other and store whichever was found inside the event variable. However in an inline event handler there isn't an e object to use. In that case you have to take advantage of the arguments collection which is always available and refers to the complete set of arguments passed to a function:

onclick="var event = arguments[0] || window.event; [...]"

However, generally speaking you should be avoiding inline event handlers if you need to to anything complicated like stopping propagation. Writing your event handlers separately and the attaching them to elements is a much better idea in the medium and long term, both for readability and maintainability.

Keep in mind that window.event is not supported in FireFox, and therefore it must be something along the lines of:

e.cancelBubble = true

Or, you can use the W3C standard for FireFox:

e.stopPropagation();

If you want to get fancy, you can do this:

function myEventHandler(e)
{
    if (!e)
      e = window.event;

    //IE9 & Other Browsers
    if (e.stopPropagation) {
      e.stopPropagation();
    }
    //IE8 and Lower
    else {
      e.cancelBubble = true;
    }
}