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The EventTarget method addEventListener sets up a function to be called whenever the specified event is delivered to the target. 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. 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 which 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. 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:. 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. This example demonstrates how to use addEventListener to watch for mouse clicks on an element. 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.
Here, we'll take a look at how to use an anonymous funtion to pass parameters into the event listener. 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.
That means that the variables and constants available to the containing function are also available to the event handler when using an arrow function.
The benefits are as follows:. The alternative, older way to register event listeners , is described below. 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.
If multiple identical EventListener s 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 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:. The value of this within modifyText is a reference to the global object Window or undefined in the case of strict mode. 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. A problem in the example above is that you cannot remove the listener with bind. Another solution is using a special function called handleEvent to catch any events:. Another way of handling the reference to this is to pass to the EventListener a function that calls the method of the object which contains the fields that need to be accessed:.
For IE, we modify the preceding example to:. 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 following code only adds IE 8 support. This IE 8 polyfill only works in standards mode: Before then, event listeners were registered as follows:. 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 normally used to register event listeners dynamically unless the extra features of addEventListener are needed.
In the first case, a new anonymous function is created with each iteration of the loop. In the second case, the same previously declared function is used as an event handler. This results in smaller memory consumption. Moreover, in the first case, it is not possible to call removeEventListener because no reference to the anonymous function is kept. In the second case, it's possible to do myElement. The default value for the passive option is false. Starting in Chrome 56 desktop, Chrome for Android, and Android webview the default value for touchstart and touchmove is true and calls to preventDefault are not permitted.
To override this behavior, you set the passive option to false as shown in the example below. This change prevents the listener from blocking page rendering while a user is scrolling. A demo is available on the Google Developer site. Please note that Edge does not support the options argument at all, and adding it will prevent the use of the useCapture argument without proper use of feature detection.
Setting passive isn't important for the basic scroll event, as it cannot be canceled, so its listener can't block page rendering anyway. Before Chrome 49, the type and listener parameters were optional. Older versions of IE supported an equivalent, proprietary, EventTarget. Get the latest and greatest from MDN delivered straight to your inbox. Please check your inbox or your spam filter for an email from us. 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.
Ideally, you should include it for the widest possible browser compatibility. The compatibility table on this page is generated from structured data. If you'd like to contribute to the data, please check out https: Form with options object supported third parameter can be either options or a Boolean , for backwards compatibility 49 49 Yes 49 Yes 10? Document Tags and Contributors Tags: Contributors to this page: Learn the best of web development Get the latest and greatest from MDN delivered straight to your inbox.