adding to window.onload event?
JavascriptJqueryJavascript Problem Overview
I'm wondering how to add another method call to the window.onload event once it has already been assigned a method call.
Suppose somewhere in the script I have this assignment...
window.onload = function(){ some_methods_1() };
and then later on in the script I have this assignment
window.onload = function(){ some_methods_2() };
As it stands, only some_methods_2
will be called. Is there any way to add to the previous window.onload
callback without cancelling some_methods_1
? (and also without including both some_methods_1()
and some_methods_2()
in the same function block).
I guess this question is not really about window.onload
but a question about javascript in general. I DON'T want to assign something to window.onload
in such a way that that if another developer were to work on the script and add a piece of code that also uses window.onload
(without looking at my previous code), he would disable my onload event.
I'm also wondering the same thing about
$(document).ready()
in jquery. How can I add to it without destroying what came before, or what might come after?
Javascript Solutions
Solution 1 - Javascript
If you are using jQuery, you don't have to do anything special. Handlers added via $(document).ready()
don't overwrite each other, but rather execute in turn:
$(document).ready(func1)
...
$(document).ready(func2)
If you are not using jQuery, you could use addEventListener
, as demonstrated by Karaxuna, plus attachEvent
for IE<9.
Note that onload
is not equivalent to $(document).ready()
- the former waits for CSS, images... as well, while the latter waits for the DOM tree only. Modern browsers (and IE since IE9) support the DOMContentLoaded
event on the document, which corresponds to the jQuery ready
event, but IE<9 does not.
if(window.addEventListener){
window.addEventListener('load', func1)
}else{
window.attachEvent('onload', func1)
}
...
if(window.addEventListener){
window.addEventListener('load', func2)
}else{
window.attachEvent('onload', func2)
}
If neither option is available (for example, you are not dealing with DOM nodes), you can still do this (I am using onload
as an example, but other options are available for onload
):
var oldOnload1=window.onload;
window.onload=function(){
oldOnload1 && oldOnload1();
func1();
}
...
var oldOnload2=window.onload;
window.onload=function(){
oldOnload2 && oldOnload2();
func2();
}
or, to avoid polluting the global namespace (and likely encountering namespace collisions), using the import/export IIFE pattern:
window.onload=(function(oldLoad){
return function(){
oldLoad && oldLoad();
func1();
}
})(window.onload)
...
window.onload=(function(oldLoad){
return function(){
oldLoad && oldLoad();
func2();
}
})(window.onload)
Solution 2 - Javascript
You can use attachEvent(ie8) and addEventListener instead
addEvent(window, 'load', function(){ some_methods_1() });
addEvent(window, 'load', function(){ some_methods_2() });
function addEvent(element, eventName, fn) {
if (element.addEventListener)
element.addEventListener(eventName, fn, false);
else if (element.attachEvent)
element.attachEvent('on' + eventName, fn);
}
Solution 3 - Javascript
There are basically two ways
-
store the previous value of
window.onload
so your code can call a previous handler if present before or after your code executes -
using the
addEventListener
approach (that of course Microsoft doesn't like and requires you to use another different name).
The second method will give you a bit more safety if another script wants to use window.onload
and does it without thinking to cooperation but the main assumption for Javascript is that all the scripts will cooperate like you are trying to do.
Note that a bad script that is not designed to work with other unknown scripts will be always able to break a page for example by messing with prototypes, by contaminating the global namespace or by damaging the dom.
Solution 4 - Javascript
This might not be a popular option, but sometimes the scripts end up being distributed in various chunks, in that case I've found this to be a quick fix
if(window.onload != null){var f1 = window.onload;}
window.onload=function(){
//do something
if(f1!=null){f1();}
}
then somewhere else...
if(window.onload != null){var f2 = window.onload;}
window.onload=function(){
//do something else
if(f2!=null){f2();}
}
this will update the onload function and chain as needed