DOM events that don't suck

npm install sensor
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sensor.js - DOM events that don't suck


  • cleaner, more efficient API for event binding
  • use as sensor(element).on('click', handler), or use sensor.patch() so you can do element.on('click', handler)
  • first-class custom events - ever wanted element.on( 'swipeleft', nextSlide );? Extensible so you can redefine events so they behave the way your app needs
  • cross-browser, cross-paradigm (mouse, touch, pointer)

The problem

The DOM has been described as the API that only a mother could love. Consider the humble click event:

el = document.getElementById( 'myButton' );
el.addEventListener( 'click', handler = function () {
  alert( 'button clicked!' );

// later...
el.removeEventListener( 'click', handler );

Yack. So much code for such a simple task! And if you want to implement event delegation, you're looking at a whole lot more. At this point you're probably thinking 'just use jQuery!'. Fine, except:

  • Not everyone wants to use a library that's 90kb minified just to gloss over the DOM's ugliness
  • The click event is broken anyway!

What do you mean, 'the click event is broken'?

Try this - open a new tab, go to about:blank, open the console, paste in window.addEventListener('click', function(){alert('clicked')});.

Then click. Hopefully the alert box popped up. Fine. Now try this - hold the mouse button down for a few seconds, and release it. Or this - hold the mouse down, waggle it about, move it from one side of the window to the other (in Firefox you can even leave the window) and back again, then release.

Did the alert box show?

In all probability, yes, it did. Now I don't know about you, but my interpretation of the word 'click' clearly differs from the W3C's (actually, they define it as 'a mousedown and mouseup over the same screen location', which merely begs the question of what qualifies as a 'screen location' - the whole thing?).

It gets worse - if your site is viewed on a touch screen, your click handlers will fire 300 milliseconds after the touchstart, touchend sequence has finished. That's lousy UX.

And it's not just click. The events the DOM gives us are indispensable, but too crude for the needs of many modern webapps.

The solution

sensor.js eases the pain in four ways. Firstly, it provides a nice API, which will look familiar to jQuery users:

// simple example
button = sensor( 'myButton' ); // or you could pass in an element, or a CSS selector
button.on( 'tap', handler = function () {
  alert( 'button tapped!' );

button.once( 'mouseover', function () {
  alert( 'moused over! you won\'t see me again, because we used button.once()' );

// later... 'tap', handler );

// or, you could do this instead - store a reference to the listener...
listener = sensor( 'myButton' ).on( 'tap', handler );

// ...then cancel it later when you need to

Secondly, it can patch Node.prototype and Window.prototype so that you don't need to call sensor( element ):


window.on( 'resize', resizeHandler );
document.getElementById( 'myButton' ).on( 'tap', handler );

(This kind of 'monkey patching' is controversial, obviously - use it at your own discretion! But let me tell you - it's goddamn liberating.)

Thirdly, it allows simple event delegation, allowing you to efficiently target multiple elements (including ones that don't yet exist):

<ul id='people'>
people = document.getElementById( 'people' );
handler = function () {
  var person = this.innerText; // `this` is the child element that matches the child selector
  alert( 'selected ' + person );

// traditional event delegation - use a CSS selector
people.on( 'tap', 'li', handler );

// alternative - use an array or array-like object (such as a NodeList)
listItems = people.querySelectorAll( 'li' ); // or people.getElementsByTagName( 'li' )

people.on( 'tap', listItems, handler )

// second alternative - use a function as the child selector
filter = function ( el ) {
  return el.tagName === 'LI';

people.on( 'tap', filter, handler );

Fourthly, it allows you to define custom events using sensor.define( eventName, eventDefinition ). sensor.js comes bundled with a number of default custom events, but you can easily add more if your app uses unique gestures, for example. (At some point I'll document how... for the time being you'll have to make do with the notes in the API section below or reverse engineer the existing examples!)



Equivalent to sensor( window ).

sensor( window )

Returns a new sensor instance on window, or returns the existing one.

sensor( element )

Returns a new sensor instance on element, or returns the existing one.

sensor( selector )

Returns a new sensor instance, or returns the existing one. selector is a CSS selector or an ID (CSS selectors, other than ID selectors such as #myButton, require document.querySelector. If you're targeting IE8 and below, bring your own polyfill).


Patches Node.prototype and Window.prototype so you can treat window and any DOM elements as though they were sensor instances

sensor.define( eventName, eventDefinition )

Defines a custom event. Thereafter, you can do e.g. listener = element.on( 'myCustomEvent', doSomething ). Event definitions take three arguments: el, elSensor, fire:

  • el - the element the event is bound to
  • elSensor - the element's sensor instance
  • fire - the function that should be called when the event is to be triggered. It should be called with the original event's target as context (to facilitate event delegation), but can have any signature (though it is wise to pass along any relevant Event objects). It must return an object with a teardown property, which is used to cancel any listeners (e.g. to underlying mousedown or touchstart events) it depended on.

Instance methods

For brevity, the following assumes that sensor.patch() has been used. If you're not that way inclined, for element read sensor instance:

element.on( eventName, handler )

Triggers handler when element is the subject of an eventName event. Within handler, this is element, and the arguments are as per the event definition (for standard events such as mousedown there is one argument - the original DOM event - in other words, you write your handlers the same way as if you were doing element.addEventListener( eventName, handler ) or $( element ).on( eventName, handler )).

Returns a listener, which is an object with a cancel method for cancelling the listener.

element.on( eventName, childSelector, handler )

Triggers handler when a descendant of element matching childSelector is the subject of an eventName event. childSelector can take one of three forms:

  • CSS selector string (tag name selectors, such as 'li', and class name selectors, such as '.item', will work everywhere. More complicated selectors depend on the presence of element.matches( selector ) or a prefixed equivalent - if you're targeting IE8 and below you will need to bring your own polyfill (here's one you could use)
  • Array or array-like object (such as a NodeList) containing the targeted elements
  • Filter function, which receives an element as its sole argument and returns a truthy value if the element is a match, a falsy value otherwise

Returns a listener, as above.

element.on( map )

Binds multiple event handlers. The following syntaxes are all valid:

  click: function () {
    alert( 'clicked element' );
  'click .item': function () {
    alert( 'clicked a descendant of element with class "item"' );
  click: {
    childSelector: element.querySelectorAll( '.button' ),
    handler: function () {
      alert( 'clicked a descendant of element with class "button"' );

Returns a listener as above, with the distinction that it will cancel all event listeners in one go.

Removes all event listeners from element (except ones that were added via another means, such as element.addEventListener( ... ) or $( element ).on( ... )). eventName )

Removes all event listeners of type eventName from element eventName, handler )

Removes all eventName listeners from element where the handler was handler, and there was no child selector. eventName, childSelector, handler )

Removes all eventName listeners from element that targeted childSelector and where the handler was handler.

element.once( eventName, handler )

Triggers handler when element is the subject of an eventName event, then cancels itself so it will be called once at most. Returns a listener.

element.once( eventName, childSelector, handler )

As above, mutatis mutandis.


Loads of stuff: Tests, complete cross-browser support, more custom events, more inline comments in the code, better documentation, and so on. If you'd like to pitch in please do! Issues and pull requests are very welcome.


I'm @rich_harris.


Copyright 2013 Rich Harris. Released under the MIT License.

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