DOM - Event

UI - Action (Interaction | Event ) in the dom.

An event allows for signaling that something has occurred, e.g., that an image has completed downloading.

Event are code structures that listen out for things happening to the document.

In the DOM specification, you can find the following:

• The type of an event
• The concept of a regular event parent and a cross-boundary event parent

The callback function that handle the event can be created with the help of:

• An event handler. Its name starts with “on” and is followed by the name of the event for which it is intended. (Example: onClick)
• An event listener

and get back an event object

The following features are defined in the DOM Events specification:

• MouseEvent interface
• MouseEventInit dictionary type
• The UIEvent interface's detail attribute
• click event

An event's type is synonymous of name type (ie “an event named click” or “if the event name is keypress”)

For example, the following document fragment:

<button class="btn btn-info" onclick="console.log('Bouh!')" >Click Me</button>

In the contexts of events, the terms fire and dispatch are used as defined in the DOM specification:

Event - Fire

Dispatching an event means to follow the steps that propagate the event through the tree.

The term trusted event is used to refer to events whose isTrusted attribute is initialized to true

Event - Missed Event

The type of an event categorize the event. It's also known as the name of the event (Ref)

Example:

• click - mouse click
• load - page load
• …

DOM - Event Type (name)

Event - Category

In the chrome devtool

An event may bubble (ie propagate) up the dom tree to other subscriber.

If you don't want to have the default behavior (such as following a link), you can prevent it (stop it)

DOM / Javascript - Event target / currentTarget

The structure of an event can be found in the Event interface definition.

Example (Credits):

• The HTML code. We will execute an event against the world span.

<p>Hello <span id="world">world</span>!</p>

• Utility function to print the node id

function getNodeId(node){
  let nodeId = node.nodeName;
  if ("id" in node){
     nodeId += "#"+node.id;
  }
  return nodeId.toLowerCase();
}

• A callback function that will give as listener parameter to be executed when the event is dispatched

function eventCallback(e) {
     console.log('-------------------');
     console.log("Target: "+getNodeId(e.target));
     console.log("Current Target: "+getNodeId(e.currentTarget));
     console.log("Event Phase: "+e.eventPhase);
}

• The custom event type (ie event name)

let eventName = "myEvent";

• Add two listeners on document and body that executes the callback for this event

document.addEventListener(eventName , eventCallback, {capture: true})
document.body.addEventListener(eventName , eventCallback)

• Create the event and dispatch it against the span world element

var myEvent = new Event(eventName, {bubbles:true})
document.getElementById("world").dispatchEvent(myEvent)

• Result:

• Document Object Model (DOM) Level 2 Events, T. Pixley. W3C.
• Document Object Model (DOM) Level 3 Events Specification, T. Leithead, J. Rossi, D. Schepers, B. Höhrmann, P. Le Hégaret, T. Pixley. W3C.

• HTML4 - Event
• HTML5 - 1.9.2 Common pitfalls to avoid when using the scripting APIs
• Progress Events, A. van Kesteren, C. McCathie Nevile; J. Song. W3C. An event interface that can be used for measuring progress; e.g. HTTP entity body transfers

• https://developer.mozilla.org/en-US/Learn/Getting_started_with_the_web/JavaScript_basics