The simple thing to do in the absence of a framework that does all the cross-browser compatibility for you is to just put a call to your code at the end of the body.
That'll be faster to execute than an onload handler because this waits only for the DOM to be ready, not for all images to load.
And, this will work in every browser.
<html>
<head>
</head>
<body>
Your HTML here
<script>
// self executing function here
(function() {
// your page initialization code here
// the DOM will be available here
})();
</script>
</body>
</html>
But, if you don't want to do this way and you need cross-browser compatibility and don't want to wait for window.onload, then you should just understand how a framework like jQuery implements its $(document).ready() method.
To give you a little idea what jQuery does (which will work wherever the script tag is placed). If supported, it tries the standard this way:
document.addEventListener('DOMContentLoaded', fn, false);
Try this with a fallback to:
window.addEventListener('load', fn, false )
Or, for older versions of IE, it uses the below code:
document.attachEvent("onreadystatechange", fn);
with a fallback to:
window.attachEvent("onload", fn);
Following is the full substitute for jQuery's .ready() written in plain javascript:
(function(funcName, baseObj) {
// The public function name defaults to window.docReady
// but you can pass in your own object and own function name and those will be used
// if you want to put them in a different namespace
funcName = funcName || "docReady";
baseObj = baseObj || window;
var readyList = [];
var readyFired = false;
var readyEventHandlersInstalled = false;
// call this when the document is ready
// this function protects itself against being called more than once
function ready() {
if (!readyFired) {
// this must be set to true before we start calling callbacks
readyFired = true;
for (var i = 0; i < readyList.length; i++) {
// if a callback here happens to add new ready handlers,
// the docReady() function will see that it already fired
// and will schedule the callback to run right after
// this event loop finishes so all handlers will still execute
// in order and no new ones will be added to the readyList
// while we are processing the list
readyList[i].fn.call(window, readyList[i].ctx);
}
// allow any closures held by these functions to free
readyList = [];
}
}
function readyStateChange() {
if ( document.readyState === "complete" ) {
ready();
}
}
// This is the one public interface
// docReady(fn, context);
// the context argument is optional - if present, it will be passed
// as an argument to the callback
baseObj[funcName] = function(callback, context) {
if (typeof callback !== "function") {
throw new TypeError("callback for docReady(fn) must be a function");
}
// if ready has already fired, then just schedule the callback
// to fire asynchronously, but right away
if (readyFired) {
setTimeout(function() {callback(context);}, 1);
return;
} else {
// add the function and context to the list
readyList.push({fn: callback, ctx: context});
}
// if document already ready to go, schedule the ready function to run
if (document.readyState === "complete") {
setTimeout(ready, 1);
} else if (!readyEventHandlersInstalled) {
// otherwise if we don't have event handlers installed, install them
if (document.addEventListener) {
// first choice is DOMContentLoaded event
document.addEventListener("DOMContentLoaded", ready, false);
// backup is window load event
window.addEventListener("load", ready, false);
} else {
// must be IE
document.attachEvent("onreadystatechange", readyStateChange);
window.attachEvent("onload", ready);
}
readyEventHandlersInstalled = true;
}
}
})("docReady", window);
For the latest version of the code, you can refer to this link: https://github.com/jfriend00/docReady
Following are the Usage:
// This is used to pass a function reference
docReady(fn);
// it is uses as an anonymous function
docReady(function() {
// code here
});
// pass a function reference and a context
// the context will be passed to the function as the first argument
docReady(fn, context);
// use an anonymous function with a context
docReady(function(context) {
// code here that can use the context argument that was passed to docReady
}, ctx);
This has been tested in:
IE6 and up
Firefox 3.6 and up
Chrome 14 and up
Safari 5.1 and up
Opera 11.6 and up
Multiple iOS devices
Multiple Android devices
Working implementation and test bed: http://jsfiddle.net/jfriend00/YfD3C/
Let's understand how it works:
1. Create an IIFE (immediately invoked function expression) so that you'll have non-public state variables.
2. Try declaring a public function docReady(fn, context)
3. When docReady(fn, context) function is called, first check if the ready handler has already fired.
4. If the ready handler is fired, then just schedule the newly added callback to fire right after this thread of JS finishes with setTimeout(fn, 1).
5. But, if the ready handler has not already fired, then add this new callback to the list of callbacks to be called later.
6. Now, check if the document is already ready. If so, execute all ready handlers.
7. But, If you haven't installed event listeners yet to know when the document becomes ready, then install them right now.
8. If document.addEventListener exists, then try to install event handlers using .addEventListener() for both "DOMContentLoaded" and "load" events. Here, the "load" is a backup event for safety and should not be needed.
9. If document.addEventListener doesn't exist, then try installing event handlers using .attachEvent() for "onreadystatechange" and "onload" events.
10. In the onreadystatechange event, check if the document.readyState === "complete" and if so, then call a function to fire all the ready handlers.
11. In all the other event handlers, call the function to fire all the ready handlers.
12. In the function to call all the ready handlers, check the state variable to see if you've already fired. If you haven't yet been called, then loop through the array of ready functions and call each one in the order they were added. Try setting a flag to indicate these have all been called so they are never executed more than once.
13. Now, clear the function array so any closures they might be using can be freed.
14. Handlers must be registered with docReady() are guaranteed to be fired in the order they were registered.
15. But, if you are trying to call docReady(fn) after the document is already ready, then the callback will be scheduled to execute as soon as the current thread of execution completes using setTimeout(fn, 1). This will allow the calling code to always assume they are async callbacks that will be called later, even if later the current thread of JS finishes and it preserves calling order.
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