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I have two js files, each one with its own window.onload handler. Depending on how I attach the two onload handlers to the window object I get a different behaviour on the second handler.
More specifically, here is my html file:
<html>
<head>
<title>Welcome to our site</title>
<script type="text/javascript" src="script1.js"> </script>
<script type="text/javascript" src="script2.js"> </script>
</head>
<body id="pageBody">
<h2 align="center">
<a href="http://www.whatever.com" id="redirect"> Wellcome to our site... c'mon in! </a>
</h2>
</body>
</html>
It loads two js files, script1.js and script2.js.
Here is the version of these two scripts that leads to the (at least by me) unexpected behaviour.
Script1.js:
window.onload = initAll1(); // attach first onload handler
function initAll1() {
alert("initAll1");
document.getElementById("redirect").onclick = foo; // attach an onclick handler
}
function foo() {
alert("we are in foo");
return false;
}
Script2.js:
addOnloadHandler(initAll2); // with this we should attach a second onload handler
function initAll2() {
alert("initAll2");
if (linkHasOnclickHandler(document.getElementById("redirect"))) {
alert("correct!");
}
else {
alert("wrong!");
}
}
function addOnloadHandler (newFunction) {
var oldevent = window.onload;
if (typeof oldevent == "function") {
window.onload = function() {
if (oldevent) {
oldevent();
}
newFunction();
};
}
else {
window.onload = newFunction;
}
}
function linkHasOnclickHandler() {
var oldevent = document.getElementById("redirect").onclick;
if (typeof oldevent == "function") {
return true;
}
else {
return false;
}
}
In Script2.js I tried to add the second onload handler in a nice noninvasive way using function addOnloadHandler(). This function does not make any assumption on whether there is already any onload handler attached to the window object. It is noninvasive because it should add the new handler without deleting previous ones.
The thing is that when loaded with addOnloadHandler(), initAll2() is not capable of detecting the fact that document.getElementById("redirect") already has foo() attached as an onclick event handler (see initAll1()). The alert message "wrong!" is triggered, which to me seems to be the wrong behaviour.
When I forget about addOnloadHandler() and attach both onload handlers in Script1.js using:
window.onload = function () {initAll1(); initAll2();};
then everything works as expected, and initAll2() launches the "correct!" alert message.
Is there something wrong about addOnloadHandler()? Could anybody make it work? I would really like to use it instead of the second method.
Thanks!
852
Just in case future people find this, and are looking for a way to use multiple event handlers when the object itself doesn't support addEventListener, attachEvent or some other form of listener stacking - i.e. it is a bespoke object, badly implemented. Then you can do the following:
object.onload = (function(pre){
return function(){
pre && pre.apply(this,arguments);
/// do what you need for your listener here
}
})(object.onload);
Each time you use the above code the previous onload listener is passed in as an argument, and when your new listener is triggered it runs the old listener first - meaning you can stack many listeners like this, if you so wish. However, this will only work for as long as the above code is always used to add listeners to your object. All your hard work will be undone if somewhere else it is overridden with a simple:
object.onload = function(){}
As a note to coders, if you are to implement a library, plugin or constructor, and it is possible other coders will take over your work. Please, please code the ability for multiple event listeners. It's really not that difficult.
183
You need to look at addEventListener and attachEvent, which are native implementations of your addOnloadHandler.
42
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