Alternate Shortcut for addEventListener() [Duplicate]
Possible Duplicate: What Is the Difference of Using addEventListener? I Noticed That Events Can Be Assigned Directly on Objects, Without Having to Use...
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Possible Duplicate:
What is the difference of using addEventListener?
I noticed that events can be assigned directly on objects, without having to use addEventListener:
document.onload = function(e){
// do stuff..
};
instead of:
document.addEventListener('load', function(e){
// do stuff..
});
So is there any reason I shouldn't use the first method? Why don't other people use it?
Also this appear to work in old IE too (in which you needed attachEvent).
Consider what happens if you try the following (I'm attaching the events to window because that is where you should listen for this event)
window.onload = function (e) {console.log('A');};
window.onload = function (e) {console.log('B');};
vs
window.addEventListener('load', function (e) {console.log('C');}, false);
window.addEventListener('load', function (e) {console.log('D');}, false);
From the first code block you will only see "B", but from the second you'll see both "C" and "D". Fiddle (please open console to see).
Apart from the fact that, as the accepted answer shows, directly binding handlers to the DOM limits the amout of handlers, addEventListener has a lot more to offer:
The event listener needn't be bound to the element directly (it doesn't have to exist in the DOM). This is useful when using ajax (think of it as jQuery's .on method).
A single listener can deal with all events of a particular type, so using event listeners requires less resources (which can speed up the overall performance)
For X-browser compatibility (like IE8), it's a lot easier to avoid memory-leaks:
window.onload = function(e)
{
alert('In IE8, this causes mem-leaks!');
};
var load = function(e)
{//cf @PaulS.'s comment & link on IE8 and symbol bleeding
e = e || window.event;//X-browser stuff
var target = e.target || e.srcElement;//so you can use this callback for all browsers
if (window.removeEventListener)
{//more X-browser stuff
return window.removeEventListener('load',load,false);
}
window.detachEvent('onload',load);//reference callback by variable name
};
if (window.addEventListener)
{
window.addEventListener('load',load,false);
}
else
{//in IE8, addEventListener doesn't exist, but it has a jScript counterpart:
//no mem-leaks in IE AFAIK
window.attachEvent('onload', load);
}
Here's a couple of links which might interest you (Yes, I know, shameless self-promotion - sorry):
Why do we need event listeners?
mem-leaks & event delegation & closures in IE8
And just for fun: a script I wrote a while back that uses event delegation and works on IE, FF, chrome, ... and touch devices. Which was a bit more tricky than I expected.
/**
* Copyright 2012, Elias Van Ootegem
* Date: Tue Jul 03 2012 +0100
*/
(function(G,undef)
{
'use strict';
var load,clickHandler,touchHandler,hide,reveal;
hide = function(elem)
{
elem.setAttribute('style','display:none;');
};
reveal = function(show,nextTo)
{
var str = 'display: block; position:relative; left:220px; top: ' + (nextTo.offsetTop - show.parentNode.offsetTop) + 'px;';
show.setAttribute('style',str);
}
load = function()
{
var doc = G.document;
if (G.removeEventListener)
{
G.removeEventListener('load',load,false);
}
else
{
G.detachEvent('onload',load);
}
if (doc.hasOwnProperty('ontouchstart'))
{//We have a touch device
touchHandler = (function(subNavs)
{
var current,divs = (function()
{
var i,r = {};
for (i=0;i<subNavs.length;i++)
{
r[subNavs[i].id] = doc.getElementById(subNavs[i].id + 'd');
hide(r[subNavs[i].id]);
}
return r;
}());
return function(e)
{
e = e || G.event;
if (e.changedTouches.length !== 1)
{//multi-touch
return e;
}
var timer,endListener,coords,target = e.target || e.srcElement;
if (target.tagName.toLowerCase() === 'img' && target.id.match(/^close[0-9]+$/))
{
hide(current);
current = undef;
return e;
}
if (target.tagName.toLowerCase() === 'a')
{
target = target.parentNode;
}
if (target.tagName.toLowerCase() !== 'p' || !target.id || !divs[target.id])
{
if (current === undef)
{
return e;
}
while(target !== doc.body)
{
target = target.parentNode;
if (target === current)
{
return e;
}
}
timer = setTimeout(function()
{
doc.body.removeEventListener('touchend',endListener,false);
clearTimeout(timer);
timer = undef;
},300);
endListener = function(e)
{
doc.body.removeEventListener('touchend',endListener,false);
clearTimeout(timer);
timer = undef;
hide(current);
current = undef;
return e;
};
return doc.body.addEventListener('touchend',endListener,false);
}
coords = {x:e.changedTouches[0].clientX,y:e.changedTouches[0].clientY};
timer = setTimeout(function()
{
doc.body.removeEventListener('touchend',endListener,false);
clearTimeout(timer);
timer = undef;
},300);
endListener = function(e)
{
e = e || G.event;
clearTimeout(timer);
timer = undef;
doc.body.removeEventListener('touchend',endListener,false);
var endCoords,endTarget = e.target || e.srcElement;
if (endTarget !== target)
{
endCoords = {x:e.changedTouches[0].clientX,y:e.changedTouches[0].clientY};
if (Math.abs(coords.x - endCoords.x) < 26 && Math.abs(coords.y - endCoords.y) < 26)
{
endTarget = target;
}
}
if (endTarget !== target)
{
return e;
}
if (current !== undef)
{
hide(current);
current = undef;
}
current = divs[target.id];
reveal(current,target);
};
doc.body.addEventListener('touchend',endListener,false);
};
}(doc.getElementsByClassName('subnavbar')));
return doc.body.addEventListener('touchstart',touchHandler,false);
}
clickHandler = (function(subNavs)
{
var current,divs = (function()
{
var i,r = {};
for (i=0;i<subNavs.length;i++)
{
r[subNavs[i].id] = doc.getElementById(subNavs[i].id + 'd');
hide(r[subNavs[i].id]);
}
return r;
}());
return function(e)
{
e = e || G.event;
var target = e.target || e.srcElement;
if (target.tagName.toLowerCase() === 'img' && target.id.match(/^close[0-9]+$/))
{
hide(current);
current = undef;
return e;
}
if (target.tagName.toLowerCase() === 'a')
{
target = target.parentNode;
}
if (target.tagName.toLowerCase() !== 'p' || !target.className.match(/\bsubnavbar\b/))
{
if (current !== undef)
{
target = (function()
{
while (target !== doc.body)
{
target = target.parentNode;
if (target === current)
{
return current;
}
}
}());
if (target !== current)
{
hide(current);
current = undef;
}
}
return e;
}
if (e.preventDefault)
{
e.preventDefault();
e.stopPropagation();
}
else
{
e.returnValue = false;
e.cancelBubble = true;
}
if (current !== undef)
{
hide(current);
}
current = divs[target.id];
reveal(current,target);
};
}(doc.getElementsByClassName('subnavbar')));
if (doc.body.addEventListener)
{
return doc.body.addEventListener('click',clickHandler,false);
}
return doc.body.attachEvent('onclick',clickHandler);
};
if (G.addEventListener)
{
return G.addEventListener('load',load,false);
}
return G.attachEvent('onload',load);
}(this));
David Miller brings 15 years of experience in global economics, personal finance strategy, and market dynamics. He specializes in turning complex economic trends into actionable insights for everyday readers.
