JS: Event Bubbling and Delegation

Learning Goals

• Understand and describe propagation and bubbling
• Understand and apply event delegation
• Utilize the event object

Vocabulary

• Event Handlers Functions that will run when an event happens
• Event Propagation Roughly, the order in which different DOM elements are notified of an event
• Event Capturing Part of the event propagation model wherein listeners are fired from the top the DOM tree, down
• Event Targeting Part of the event propagation model wherein listeners are fired on the source of the event
• Event Bubbling Part of the event propagation model wherein listeners are fired from the target of the event, up
• Event Delegation The process of using event propagation to handle events at a higher level in the DOM than the element on which the event originated

Event Basics

Events are happening all the time in the browser. When the browser has finished loading the page, an event is fired. Every time the user moves their mouse, hovers over an element, clicks or taps, submits a form, presses down on a key or takes their finger off that key — an event is fired. Some of these events are very easy to spot when they occur (e.g. the user clicks on a hyperlink), but many go by completely unnoticed.

It is, however, possible for us to use JavaScript to set up handlers for events that interest us. Our handlers wait patiently on a DOM node until the event they’re waiting for is fired. Then, they spring into action, running an appropriate function to respond to the event as required, or whatever else you deem appropriate.

For a review of how to set event handlers, please refer to the DOM Manipulation with JavaScript Lesson

The Problem with Dynamic Elements

A common obstacle that many JavaScript developers struggle with is understanding the timing in which they bind event listeners to DOM nodes. When we add event listeners to DOM nodes, we’re only adding them to the nodes that are currently on the page. We are not adding listeners to nodes that may be added to the page in the future.

Experiment

Clone this repository to your local machine, and open up example1/index.html in your browser. Additionally, open up the example1/script.js and example1/index.html in your text editor, side by side.

Event Propagation

Event propagation is an important yet misunderstood topic/term when talking about events. Event propagation is an overarching term that includes the three different phases of DOM Events: capturing, targeting, and bubbling. Event propagation is bi-directional (starts at the window… goes to the target… and ends at the window) and is often improperly used as a synonym for event bubbling. Every time an event occurs, event propagation is occurring behind the scenes.

• Event capture phase - When an event occurs in the DOM, notification of the event is passed starting at the top of the DOM tree and passing down through all parent element nodes all the way to the target node where the event occurred.
• Event target phase - After the capturing phase occurs, the Target phase occurs. The target phase only includes a notification of Node where the event took place.
• Event bubbling phase - This is the final phase to occur, although many people think this is the first phase. In the bubbling phase a notice is passed from the target Node up through all of the parent Nodes all the way back to the top root of the DOM.
• OK but what is actually happening during these phases? When an event occurs - JavaScript/the browser start at the ‘top’ of the DOM tree then looks down/zooms into the target (whatever element the event occurred on), then looks back up/out element by element to see if there are any event listeners/handlers that need fired

Event Bubbling

Now we’ve talked about the fundamentals of events, let’s turn our attention to the event bubbling phase, which refers to the ability of events that occur on DOM nodes to “bubble up” and also apply to ancestors of those nodes.

  grandParent.addEventListener('click', function() {
    console.log('Grandparent');
  });

  parent.addEventListener('click', function() {
    console.log('Parent');
  });

  button.addEventListener('click', function() {
    console.log('Button');
  });

The Event Object

Sometimes inside an event handler function, you might see a parameter specified with a name such as event, evt, or simply e. This parameter holds the event object, and it is automatically passed to event handlers to provide extra features and information. In the case of the click event we’ve been using as an example, this is a MouseEvent. You can visit the MDN page for Event to explore the full list of supported event types.

Each type of event supports a number of different properties. MouseEvents contain information about the x and y coordinates where the mouse was clicked. KeyboardEvent has information about which key was pressed. The target property on the Event object can be useful during the event bubbling phase.

Event Delegation

With the understanding of event propagation and the event object, we can now go back to the original issue with handling events on dynamic content. Rather than manage the addition and removal of event listeners, there is a methodology you can use called event delegation.

In event delegation, we take advantage of the fact that events bubble in the event loops by setting an event listener on one parent. This event listener analyzes bubbled events to find a match in its child elements. Event delegation is one of the most helpful patterns for DOM events. It simplifies things and can save memory since there is no need to add many handlers.

The steps:

1. Put a single handler (event listener and function) on a parent container/area
2. In our function, check the source element (whichever element the event directly occurred on) using event.target[some property] (whatever property we can use to identify the element(s) we care about)
3. If the event happened on an element that we care about, THEN ‘handle’ that event (aka, do all the behaviors outlined in the code in the function). Remember to use console.log(event) to figure out what info you have to work with for whichever element. Remember that the ‘target’ will depend on where/what you ‘click’ on.

var parent = document.querySelector('.parent');

parent.addEventListener('click', function(event) {
  if (event.target.className === 'click-me') {
    // do your action on your 'button' or whatever it is you're listening for
  }
});

Applying Event Bubbling & Event Delegation In Your Projects

• Think about which larger container/area/parent you should put your listener on
  • Listen to this whole area (and everything in it) for the specific type of event. Ex. ‘click’
• Think about which element(s) you actually care about being clicked on
  • How can they be identified? Do they have a specific class/id we can look for?
• Think about what you want to happen when one of those elements-of-interest is clicked on
  • What should happen?
  • Which element should it happen to? The one that was clicked or a relative?
    • How can you tell JS which element you want the thing to happen to?

Resources

• A very detailed piece that describes event order can be found here