14.10 DOM Event Listener

Add an Event Handler to an Element

Example 2: JS addEventListener() part 2

Alert "Hello World!" when the user clicks on an element:

element.addEventListener("click", function(){ alert("Hello World!"); });

You can also refer to an external "named" function:

Example 3: JS addEventListener() part 3

Alert "Hello World!" when the user clicks on an element:

element.addEventListener("click", myFunction);

function myFunction() {
  alert ("Hello World!");
}

Add Many Event Handlers to the Same Element

The addEventListener() method allows you to add many events to the same element, without overwriting existing events:

Example 4: JS addEventListener() part 4

element.addEventListener("click", myFunction);
element.addEventListener("click", mySecondFunction);

You can add events of different types to the same element:

Example 5: JS addEventListener() part 5

element.addEventListener("mouseover", myFunction);
element.addEventListener("click", mySecondFunction);
element.addEventListener("mouseout", myThirdFunction);

Add an Event Handler to the window Object

The addEventListener() method allows you to add event listeners on any HTML DOM object such as HTML elements, the HTML document, the window object, or other objects that support events, like the xmlHttpRequest object.

Example 6: JS addEventListener() part 6

Add an event listener that fires when a user resizes the window:

window.addEventListener("resize", function(){
  document.getElementById("demo").innerHTML = sometext;
});

Passing Parameters

When passing parameter values, use an "anonymous function" that calls the specified function with the parameters:

Example 7: JS addEventListener() part 7

element.addEventListener("click", function(){ myFunction(p1, p2); });

Event Bubbling or Event Capturing?

• There are two ways of event propagation in the HTML DOM, bubbling and capturing.
• Event propagation is a way of defining the element order when an event occurs. If you have a <p> element inside a <div> element, and the user clicks on the <p> element, which element's "click" event should be handled first?
• In bubbling the inner most element's event is handled first and then the outer: the <p> element's click event is handled first, then the <div> element's click event.
• In capturing the outer most element's event is handled first and then the inner: the <div> element's click event will be handled first, then the <p> element's click event.
• With the addEventListener() method you can specify the propagation type by using the "useCapture" parameter:

addEventListener(event, function, useCapture);

The default value is false, which will use the bubbling propagation, when the value is set to true, the event uses the capturing propagation.

Example 8: JS addEventListener() part 8

document.getElementById("myP").addEventListener("click", myFunction, true);
document.getElementById("myDiv").addEventListener("click", myFunction, true);

The removeEventListener() method

The removeEventListener() method removes event handlers that have been attached with the addEventListener() method:

Example 9: JS removeEventListener()

element.removeEventListener("mousemove", myFunction);