Question

Event handlers registered on nodes with children will also receive some events that happen in the children. If a button inside a paragraph is clicked, event handlers on the paragraph will also receive the click event.

But if both the paragraph and the button have a handler, the more specific handler—the one on the button—gets to go first. The event is said to propagate outward, from the node where it happened to that node’s parent node and on to the root of the document. Finally, after all handlers registered on a specific node have had their turn, handlers registered on the whole window get a chance to respond to the event.

At any point, an event handler can call the stopPropagation method on the event object to prevent handlers “further up” from receiving the event. This can be useful when, for example, you have a button inside another clickable element and you don’t want clicks on the button to activate the outer element’s click behavior.

The following example registers "mousedown" handlers on both a button and the paragraph around it. When clicked with the right mouse button, the handler for the button calls stopPropagation , which will prevent the handler on the paragraph from running. When the button is clicked with another mouse button, both handlers will run.

<p>A paragraph with a <button>button</button>.</p>
<script>
  var para = document.querySelector("p");
  var button = document.querySelector("button");
  para.addEventListener("mousedown", function() {
    console.log("Handler for paragraph.");
  });
  button.addEventListener("mousedown", function(event) {
    console.log("Handler for button.");
    if (event.which == 3)
      event.stopPropagation();
  });
</script>

Most event objects have a target property that refers to the node where they originated. You can use this property to ensure that you’re not accidentally handling something that propagated up from a node you do not want to handle.

It is also possible to use the target property to cast a wide net for a specific type of event. For example, if you have a node containing a long list of buttons, it may be more convenient to register a single click handler on the outer node and have it use the target property to figure out whether a button was clicked, rather than register individual handlers on all of the buttons.

<button>A</button>
<button>B</button>
<button>C</button>
<script>
  document.body.addEventListener("click", function(event) {
    if (event.target.nodeName == "BUTTON")
      console.log("Clicked", event.target.textContent);
  });
</script>

This is a book about getting computers to do what you want them to do. Computers are about as common as screwdrivers today, but they contain a lot more hidden complexity and thus are harder to operate and understand. To many, they remain alien, slightly threatening things.