EventTarget.addEventListener() - Web APIs 编辑

The EventTarget method addEventListener() sets up a function that will be called whenever the specified event is delivered to the target. Common targets are Element, Document, and Window, but the target may be any object that supports events (such as XMLHttpRequest).

addEventListener() works by adding a function or an object that implements EventListener to the list of event listeners for the specified event type on the EventTarget on which it's called.

Syntax

target.addEventListener(type, listener [, options]);
target.addEventListener(type, listener [, useCapture]);
target.addEventListener(type, listener [, useCapture, wantsUntrusted ]); // Gecko/Mozilla only

Parameters

type

A case-sensitive string representing the event type to listen for.

listener

The object that receives a notification (an object that implements the Event interface) when an event of the specified type occurs. This must be an object implementing the EventListener interface, or a JavaScript function. See The event listener callback for details on the callback itself.

options Optional

An options object specifies characteristics about the event listener. The available options are:

capture

A Boolean indicating that events of this type will be dispatched to the registered listener before being dispatched to any EventTarget beneath it in the DOM tree.

once

A Boolean indicating that the listener should be invoked at most once after being added. If true, the listener would be automatically removed when invoked.

passive

A Boolean that, if true, indicates that the function specified by listener will never call preventDefault(). If a passive listener does call preventDefault(), the user agent will do nothing other than generate a console warning. See Improving scrolling performance with passive listeners to learn more.

mozSystemGroup

A Boolean indicating that the listener should be added to the system group. Available only in code running in XBL or in the chrome of the Firefox browser.

useCapture Optional

A Boolean indicating whether events of this type will be dispatched to the registered listener before being dispatched to any EventTarget beneath it in the DOM tree. Events that are bubbling upward through the tree will not trigger a listener designated to use capture. Event bubbling and capturing are two ways of propagating events that occur in an element that is nested within another element, when both elements have registered a handle for that event. The event propagation mode determines the order in which elements receive the event. See DOM Level 3 Events and JavaScript Event order for a detailed explanation. If not specified, useCapture defaults to false.

Note: For event listeners attached to the event target, the event is in the target phase, rather than the capturing and bubbling phases. Events in the target phase will trigger all listeners on an element in the order they were registered, regardless of the useCapture parameter. Note: useCapture has not always been optional. Ideally, you should include it for the widest possible browser compatibility.

wantsUntrusted

A Firefox (Gecko)-specific parameter. If true, the listener receives synthetic events dispatched by web content (the default is false for browser chrome and true for regular web pages). This parameter is useful for code found in add-ons, as well as the browser itself.

Return value

undefined

Usage notes

The event listener callback

The event listener can be specified as either a callback function or an object that implements EventListener, whose handleEvent() method serves as the callback function.

The callback function itself has the same parameters and return value as the handleEvent() method; that is, the callback accepts a single parameter: an object based on Event describing the event that has occurred, and it returns nothing.

For example, an event handler callback that can be used to handle both fullscreenchange and fullscreenerror might look like this:

function eventHandler(event) {
  if (event.type == 'fullscreenchange') {
    /* handle a full screen toggle */
  } else /* fullscreenerror */ {
    /* handle a full screen toggle error */
  }
}

Safely detecting option support

In older versions of the DOM specification, the third parameter of addEventListener() was a Boolean value indicating whether or not to use capture. Over time, it became clear that more options were needed. Rather than adding more parameters to the function (complicating things enormously when dealing with optional values), the third parameter was changed to an object that can contain various properties defining the values of options to configure the process of removing the event listener.

Because older browsers (as well as some not-too-old browsers) still assume the third parameter is a Boolean, you need to build your code to handle this scenario intelligently. You can do this by using feature detection for each of the options you're interested in.

For example, if you want to check for the passive option:

let passiveSupported = false;

try {
  const options = {
    get passive() { // This function will be called when the browser
                    //   attempts to access the passive property.
      passiveSupported = true;
      return false;
    }
  };

  window.addEventListener("test", null, options);
  window.removeEventListener("test", null, options);
} catch(err) {
  passiveSupported = false;
}

This creates an options object with a getter function for the passive property; the getter sets a flag, passiveSupported, to true if it gets called. That means that if the browser checks the value of the passive property on the options object, passiveSupported will be set to true; otherwise, it will remain false. We then call addEventListener() to set up a fake event handler, specifying those options, so that the options will be checked if the browser recognizes an object as the third parameter. Then, we call removeEventListener() to clean up after ourselves. (Note that handleEvent() is ignored on event listeners that aren't called.)

You can check whether any option is supported this way. Just add a getter for that option using code similar to what is shown above.

Then, when you want to create an actual event listener that uses the options in question, you can do something like this:

someElement.addEventListener("mouseup", handleMouseUp, passiveSupported
                               ? { passive: true } : false);

Here we're adding a listener for the mouseup event on the element someElement. For the third parameter, if passiveSupported is true, we're specifying an options object with passive set to true; otherwise, we know that we need to pass a Boolean, and we pass false as the value of the useCapture parameter.

If you'd prefer, you can use a third-party library like Modernizr or Detect It to do this test for you.

You can learn more from the article about EventListenerOptions from the Web Incubator Community Group.

Examples

Add a simple listener

This example demonstrates how to use addEventListener() to watch for mouse clicks on an element.

HTML

<table id="outside">
  <tr><td id="t1">one</td></tr>
  <tr><td id="t2">two</td></tr>
</table>

JavaScript

// Function to change the content of t2
function modifyText() {
  const t2 = document.getElementById("t2");
  if (t2.firstChild.nodeValue == "three") {
    t2.firstChild.nodeValue = "two";
  } else {
    t2.firstChild.nodeValue = "three";
  }
}

// Add event listener to table
const el = document.getElementById("outside");
el.addEventListener("click", modifyText, false);

In this code, modifyText() is a listener for click events registered using addEventListener(). A click anywhere in the table bubbles up to the handler and runs modifyText().

Result

Event listener with anonymous function

Here, we'll take a look at how to use an anonymous function to pass parameters into the event listener.

HTML

<table id="outside">
  <tr><td id="t1">one</td></tr>
  <tr><td id="t2">two</td></tr>
</table>

JavaScript

// Function to change the content of t2
function modifyText(new_text) {
  const t2 = document.getElementById("t2");
  t2.firstChild.nodeValue = new_text;
}

// Function to add event listener to table
const el = document.getElementById("outside");
el.addEventListener("click", function(){modifyText("four")}, false);

Notice that the listener is an anonymous function that encapsulates code that is then, in turn, able to send parameters to the modifyText() function, which is responsible for actually responding to the event.

Result

Event listener with an arrow function

This example demonstrates a simple event listener implemented using arrow function notation.

HTML

<table id="outside">
  <tr><td id="t1">one</td></tr>
  <tr><td id="t2">two</td></tr>
</table>

JavaScript

// Function to change the content of t2
function modifyText(new_text) {
  const t2 = document.getElementById("t2");
  t2.firstChild.nodeValue = new_text;
}

// Add event listener to table with an arrow function
const el = document.getElementById("outside");
el.addEventListener("click", () => { modifyText("four"); }, false);

Result

Please note that while anonymous and arrow functions are similar, they have different this bindings. While anonymous (and all traditional JavaScript functions) create their own this bindings, arrow functions inherit the this binding of the containing function.

That means that the variables and constants available to the containing function are also available to the event handler when using an arrow function.

Example of options usage

HTML

<div class="outer">
  outer, once & none-once
  <div class="middle" target="_blank">
    middle, capture & none-capture
    <a class="inner1" href="https://www.mozilla.org" target="_blank">
      inner1, passive & preventDefault(which is not allowed)
    </a>
    <a class="inner2" href="/wiki/" target="_blank">
      inner2, none-passive & preventDefault(not open new page)
    </a>
  </div>
</div>

CSS

.outer, .middle, .inner1, .inner2 {
  display: block;
  width:   520px;
  padding: 15px;
  margin:  15px;
  text-decoration: none;
}
.outer {
  border: 1px solid red;
  color:  red;
}
.middle {
  border: 1px solid green;
  color:  green;
  width:  460px;
}
.inner1, .inner2 {
  border: 1px solid purple;
  color:  purple;
  width:  400px;
}

JavaScript

const outer  = document.querySelector('.outer');
const middle = document.querySelector('.middle');
const inner1 = document.querySelector('.inner1');
const inner2 = document.querySelector('.inner2');

const capture = {
  capture : true
};
const noneCapture = {
  capture : false
};
const once = {
  once : true
};
const noneOnce = {
  once : false
};
const passive = {
  passive : true
};
const nonePassive = {
  passive : false
};

outer.addEventListener('click', onceHandler, once);
outer.addEventListener('click', noneOnceHandler, noneOnce);
middle.addEventListener('click', captureHandler, capture);
middle.addEventListener('click', noneCaptureHandler, noneCapture);
inner1.addEventListener('click', passiveHandler, passive);
inner2.addEventListener('click', nonePassiveHandler, nonePassive);

function onceHandler(event) {
  alert('outer, once');
}
function noneOnceHandler(event) {
  alert('outer, none-once, default');
}
function captureHandler(event) {
  //event.stopImmediatePropagation();
  alert('middle, capture');
}
function noneCaptureHandler(event) {
  alert('middle, none-capture, default');
}
function passiveHandler(event) {
  // Unable to preventDefault inside passive event listener invocation.
  event.preventDefault();
  alert('inner1, passive, open new page');
}
function nonePassiveHandler(event) {
  event.preventDefault();
  //event.stopPropagation();
  alert('inner2, none-passive, default, not open new page');
}

Result

Click the outer, middle, inner containers respectively to see how the options work.

Before using a particular value in the options object, it's a good idea to ensure that the user's browser supports it, since these are an addition that not all browsers have supported historically. See Safely detecting option support for details.

Other notes

Why use addEventListener()?

addEventListener() is the way to register an event listener as specified in W3C DOM. The benefits are as follows:

• It allows adding more than a single handler for an event. This is particularly useful for AJAX libraries, JavaScript modules, or any other kind of code that needs to work well with other libraries/extensions.
• It gives you finer-grained control of the phase when the listener is activated (capturing vs. bubbling).
• It works on any DOM element, not just HTML elements.

The alternative, older way to register event listeners, is described below.

Adding a listener during event dispatch

If an EventListener is added to an EventTarget while it is processing an event, that event does not trigger the listener. However, that same listener may be triggered during a later stage of event flow, such as the bubbling phase.

Multiple identical event listeners

If multiple identical EventListeners are registered on the same EventTarget with the same parameters, the duplicate instances are discarded. They do not cause the EventListener to be called twice, and they do not need to be removed manually with the removeEventListener() method.

Note, however that when using an anonymous function as the handler, such listeners will NOT be identical, because anonymous functions are not identical even if defined using the SAME unchanging source-code called repeatedly, even if in a loop.

However, repeatedly defining the same named function in such cases can be more problematic. (See Memory issues, below.)

The value of "this" within the handler

It is often desirable to reference the element on which the event handler was fired, such as when using a generic handler for a set of similar elements.

If attaching a handler function to an element using addEventListener(), the value of this inside the handler is a reference to the element. It is the same as the value of the currentTarget property of the event argument that is passed to the handler.

my_element.addEventListener('click', function (e) {
  console.log(this.className)           // logs the className of my_element
  console.log(e.currentTarget === this) // logs `true`
})

As a reminder, arrow functions do not have their own this context.

my_element.addEventListener('click', (e) => {
  console.log(this.className)           // WARNING: `this` is not `my_element`
  console.log(e.currentTarget === this) // logs `false`
})

If an event handler (for example, onclick) is specified on an element in the HTML source, the JavaScript code in the attribute value is effectively wrapped in a handler function that binds the value of this in a manner consistent with the addEventListener(); an occurrence of this within the code represents a reference to the element.

<table id="my_table" onclick="console.log(this.id);"><!-- `this` refers to the table; logs 'my_table' -->
  ...
</table>

Note that the value of this inside a function, called by the code in the attribute value, behaves as per standard rules. This is shown in the following example:

<script>
  function logID() { console.log(this.id); }
</script>
<table id="my_table" onclick="logID();"><!-- when called, `this` will refer to the global object -->
  ...
</table>

The value of this within logID() is a reference to the global object Window (or undefined in the case of strict mode.

Specifying "this" using bind()

The Function.prototype.bind() method lets you specify the value that should be used as this for all calls to a given function. This lets you easily bypass problems where it's unclear what this will be, depending on the context from which your function was called. Note, however, that you'll need to keep a reference to the listener around so you can remove it later.

This is an example with and without bind():

const Something = function(element) {
  // |this| is a newly created object
  this.name = 'Something Good';
  this.onclick1 = function(event) {
    console.log(this.name); // undefined, as |this| is the element
  };
  this.onclick2 = function(event) {
    console.log(this.name); // 'Something Good', as |this| is bound to newly created object
  };
  element.addEventListener('click', this.onclick1, false);
  element.addEventListener('click', this.onclick2.bind(this), false); // Trick
}
const s = new Something(document.body);

Another solution is using a special function called handleEvent() to catch any events:

const Something = function(element) {
  // |this| is a newly created object
  this.name = 'Something Good';
  this.handleEvent = function(event) {
    console.log(this.name); // 'Something Good', as this is bound to newly created object
    switch(event.type) {
      case 'click':
        // some code here...
        break;
      case 'dblclick':
        // some code here...
        break;
    }
  };

  // Note that the listeners in this case are |this|, not this.handleEvent
  element.addEventListener('click', this, false);
  element.addEventListener('dblclick', this, false);

  // You can properly remove the listeners
  element.removeEventListener('click', this, false);
  element.removeEventListener('dblclick', this, false);
}
const s = new Something(document.body);

Another way of handling the reference to this is to pass to the EventListener a function that calls the method of the object that contains the fields that need to be accessed:

class SomeClass {

  constructor() {
    this.name = 'Something Good';
  }

  register() {
    const that = this;
    window.addEventListener('keydown', function(e) { that.someMethod(e); });
  }

  someMethod(e) {
    console.log(this.name);
    switch(e.keyCode) {
      case 5:
        // some code here...
        break;
      case 6:
        // some code here...
        break;
    }
  }

}

const myObject = new SomeClass();
myObject.register();

Getting data into and out of an event listener

It may seem that event listeners are like islands, and that it is extremely difficult to pass them any data, much less to get any data back from them after they execute. Event listeners only take one argument, the Event Object, which is automatically passed to the listener, and the return value is ignored. So how can we get data in and back out of them again? There are a number of good methods for doing this.

Getting data into an event listener using "this"

As mentioned above, you can use Function.prototype.bind() to pass a value to an event listener via the this reference variable.

const myButton = document.getElementById('my-button-id');
const someString = 'Data';

myButton.addEventListener('click', function () {
  console.log(this);  // Expected Value: 'Data'
}.bind(someString));

This method is suitable when you don't need to know which HTML element the event listener fired on programmatically from within the event listener. The primary benefit to doing this is that the event listener receives the data in much the same way that it would if you were to actually pass it through its argument list.

Getting data into an event listener using the outer scope property

When an outer scope contains a variable declaration (with const, let), all the inner functions declared in that scope have access to that variable (look here for information on outer/inner functions, and here for information on variable scope). Therefore, one of the simplest ways to access data from outside of an event listener is to make it accessible to the scope in which the event listener is declared.

const myButton = document.getElementById('my-button-id');
const someString = 'Data';

myButton.addEventListener('click', function() {
  console.log(someString);  // Expected Value: 'Data'

  someString = 'Data Again';
});

console.log(someString);  // Expected Value: 'Data' (will never output 'Data Again')

Note: Although inner scopes have access to const, let variables from outer scopes, you cannot expect any changes to these variables to be accessible after the event listener definition, within the same outer scope. Why? Because by the time the event listener would execute, the scope in which it was defined would have already finished executing.

Getting data into and out of an event listener using objects

Unlike most functions in JavaScript, objects are retained in memory as long as a variable referencing them exists in memory. This, and the fact that objects can have properties, and that they can be passed around by reference, makes them likely candidates for sharing data among scopes. Let's explore this.

Note: Functions in JavaScript are actually objects. (Hence they too can have properties, and will be retained in memory even after they finish executing if assigned to a variable that persists in memory.)

Because object properties can be used to store data in memory as long as a variable referencing the object exists in memory, you can actually use them to get data into an event listener, and any changes to the data back out after an event handler executes. Consider this example.

const myButton = document.getElementById('my-button-id');
const someObject = {aProperty: 'Data'};

myButton.addEventListener('click', function() {
  console.log(someObject.aProperty);  // Expected Value: 'Data'

  someObject.aProperty = 'Data Again';  // Change the value
});

window.setInterval(function() {
  if (someObject.aProperty === 'Data Again') {
    console.log('Data Again: True');
    someObject.aProperty = 'Data';  // Reset value to wait for next event execution
  }
}, 5000);

In this example, even though the scope in which both the event listener and the interval function are defined would have finished executing before the original value of someObject.aProperty would have changed, because someObject persists in memory (by reference) in both the event listener and interval function, both have access to the same data (i.e. when one changes the data, the other can respond to the change).

Note: Objects are stored in variables by reference, meaning only the memory location of the actual data is stored in the variable. Among other things, this means variables that "store" objects can actually affect other variables that get assigned ("store") the same object reference. When two variables reference the same object (e.g., let a = b = {aProperty: 'Yeah'};), changing the data in either variable will affect the other.

Note: Because objects are stored in variables by reference, you can return an object from a function to keep it alive (preserve it in memory so you don't lose the data) after that function stops executing.

Legacy Internet Explorer and attachEvent

In Internet Explorer versions before IE 9, you have to use attachEvent(), rather than the standard addEventListener(). For IE, we modify the preceding example to:

if (el.addEventListener) {
  el.addEventListener('click', modifyText, false);
} else if (el.attachEvent)  {
  el.attachEvent('onclick', modifyText);
}

There is a drawback to attachEvent(): The value of this will be a reference to the window object, instead of the element on which it was fired.

The attachEvent() method could be paired with the onresize event to detect when certain elements in a web page were resized. The proprietary mselementresize event, when paired with the addEventListener method of registering event handlers, provides similar functionality as onresize, firing when certain HTML elements are resized.

Polyfill

You can work around addEventListener(), removeEventListener(), Event.preventDefault(), and Event.stopPropagation() not being supported by Internet Explorer 8 by using the following code at the beginning of your script. The code supports the use of handleEvent() and also the DOMContentLoaded event.

Note: useCapture is not supported, as IE 8 does not have any alternative method. The following code only adds IE 8 support. This IE 8 polyfill only works in standards mode: a doctype declaration is required.

(function() {
  if (!Event.prototype.preventDefault) {
    Event.prototype.preventDefault=function() {
      this.returnValue=false;
    };
  }
  if (!Event.prototype.stopPropagation) {
    Event.prototype.stopPropagation=function() {
      this.cancelBubble=true;
    };
  }
  if (!Element.prototype.addEventListener) {
    var eventListeners=[];

    var addEventListener=function(type,listener /*, useCapture (will be ignored) */) {
      var self=this;
      var wrapper=function(e) {
        e.target=e.srcElement;
        e.currentTarget=self;
        if (typeof listener.handleEvent != 'undefined') {
          listener.handleEvent(e);
        } else {
          listener.call(self,e);
        }
      };
      if (type=="DOMContentLoaded") {
        var wrapper2=function(e) {
          if (document.readyState=="complete") {
            wrapper(e);
          }
        };
        document.attachEvent("onreadystatechange",wrapper2);
        eventListeners.push({object:this,type:type,listener:listener,wrapper:wrapper2});

        if (document.readyState=="complete") {
          var e=new Event();
          e.srcElement=window;
          wrapper2(e);
        }
      } else {
        this.attachEvent("on"+type,wrapper);
        eventListeners.push({object:this,type:type,listener:listener,wrapper:wrapper});
      }
    };
    var removeEventListener=function(type,listener /*, useCapture (will be ignored) */) {
      var counter=0;
      while (counter<eventListeners.length) {
        var eventListener=eventListeners[counter];
        if (eventListener.object==this && eventListener.type==type && eventListener.listener==listener) {
          if (type=="DOMContentLoaded") {
            this.detachEvent("onreadystatechange",eventListener.wrapper);
          } else {
            this.detachEvent("on"+type,eventListener.wrapper);
          }
          eventListeners.splice(counter, 1);
          break;
        }
        ++counter;
      }
    };
    Element.prototype.addEventListener=addEventListener;
    Element.prototype.removeEventListener=removeEventListener;
    if (HTMLDocument) {
      HTMLDocument.prototype.addEventListener=addEventListener;
      HTMLDocument.prototype.removeEventListener=removeEventListener;
    }
    if (Window) {
      Window.prototype.addEventListener=addEventListener;
      Window.prototype.removeEventListener=removeEventListener;
    }
  }
})();

Older way to register event listeners

addEventListener() was introduced with the DOM 2 Events specification. Before then, event listeners were registered as follows:

// Passing a function reference — do not add '()' after it, which would call the function!
el.onclick = modifyText;

// Using a function expression
element.onclick = function() {
  // ... function logic ...
};

This method replaces the existing click event listener(s) on the element if there are any. Other events and associated event handlers such as blur (onblur) and keypress (onkeypress) behave similarly.

Because it was essentially part of DOM 0, this technique for adding event listeners is very widely supported and requires no special cross-browser code. It is used to register event listeners dynamically when very old browsers (like IE <=8) must be supported; see the table below for details on browser support for addEventListener.

Memory issues

const els = document.getElementsByTagName('*');

// Case 1
for(let i=0 ; i < els.length; i++){
  els[i].addEventListener("click", function(e){/*do something*/}, false);
}

// Case 2
function processEvent(e){
  /* do something */
}

for(let i=0 ; i < els.length; i++){
  els[i].addEventListener("click", processEvent, false);
}

In the first case above, a new (anonymous) handler function is created with each iteration of the loop. In the second case, the same previously declared function is used as an event handler, which results in smaller memory consumption because there is only one handler function created. Moreover, in the first case, it is not possible to call removeEventListener() because no reference to the anonymous function is kept (or here, not kept to any of the multiple anonymous functions the loop might create.) In the second case, it's possible to do myElement.removeEventListener("click", processEvent, false) because processEvent is the function reference.

Actually, regarding memory consumption, the lack of keeping a function reference is not the real issue; rather it is the lack of keeping a STATIC function reference. In both problem-cases below, a function reference is kept, but since it is redefined on each iteration, it is not static. In the third case, the reference to the anonymous function is being reassigned with each iteration. In the fourth case, the entire function definition is unchanging, but it is still being repeatedly defined as if new (unless it was [[promoted]] by the compiler) and so is not static. Therefore, though appearing to be [[Multiple identical event listeners]], in both cases each iteration will instead create a new listener with its own unique reference to the handler function. However, since the function definition itself does not change, the SAME function may still be called for every duplicate listener (especially if the code gets optimized.)

Also in both cases, because the function reference was kept but repeatedly redefined with each add, the remove-statement from above can still remove a listener, but now only the last one added.

// For illustration only: Note "MISTAKE" of [j] for [i] thus causing desired events to all attach to SAME element

// Case 3
for(let i=0, j=0 ; i<els.length ; i++){
  /* do lots of stuff with j */
  els[j].addEventListener("click", processEvent = function(e){/*do something*/}, false);
}

// Case 4
for(let i=0, j=0 ; i<els.length ; i++){
  /* do lots of stuff with j */
  function processEvent(e){/*do something*/};
  els[j].addEventListener("click", processEvent, false);
}

Improving scrolling performance with passive listeners

According to the specification, the default value for the passive option is always false. However, this introduces the potential for event listeners handling certain touch events (among others) to block the browser's main thread while it is attempting to handle scrolling, resulting in possibly enormous reduction in performance during scroll handling.

To prevent this problem, some browsers (specifically, Chrome and Firefox) have changed the default value of the passive option to true for the touchstart and touchmove events on the document-level nodes Window, Document, and Document.body. This prevents the event listener from being called, so it can't block page rendering while the user is scrolling.

Note: See the compatibility table below if you need to know which browsers (and/or which versions of those browsers) implement this altered behavior.

You can override this behavior by explicitly setting the value of passive to false, as shown here:

/* Feature detection */
let passiveIfSupported = false;

try {
  window.addEventListener("test", null,
    Object.defineProperty(
      {},
      "passive",
      {
        get: function() { passiveIfSupported = { passive: false }; }
      }
    )
  );
} catch(err) {}

window.addEventListener('scroll', function(event) {
  /* do something */
  // can't use event.preventDefault();
}, passiveIfSupported );

On older browsers that don't support the options parameter to addEventListener(), attempting to use it prevents the use of the useCapture argument without proper use of feature detection.

You don't need to worry about the value of passive for the basic scroll event. Since it can't be canceled, event listeners can't block page rendering anyway.

Specifications

Browser compatibility

BCD tables only load in the browser

See also

• EventTarget.removeEventListener()
• Creating and triggering custom events
• More details on the use of this in event handlers