Pooja in Web Development

With jQuery, we all know the wonderful .ready() function:

$('document').ready(function(){});

However, let's say I want to run a function that is written in standard JavaScript with no library backing it, and that I want to launch a function as soon as the page is ready to handle it. What's the proper way to approach this?

I know I can do:

window.onload="myFunction()";

Or I can use the body tag:

<body onload="myFunction()">

Or I can even try at the bottom of the page after everything, but the end body or html tag like:

<script type="text/javascript">
 myFunction();
</script>

What is a cross-browser(old/new)-compliant method of issuing one or more functions in a manner like jQuery's $.ready()?

4 Answers

0 votes
Nadira

The simplest 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. This is 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 works in every browser.

<!doctype html>
<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>

For modern browsers (anything from IE9 and newer and any version of Chrome, Firefox or Safari), if you want to be able to implement a jQuery like $(document).ready() method that you can call from anywhere (without worrying about where the calling script is positioned), you can just use something like this:

function docReady(fn) {
 // see if DOM is already available
 if (document.readyState === "complete" || document.readyState === "interactive") {
 // call on next available tick
 setTimeout(fn, 1);
 } else {
 document.addEventListener("DOMContentLoaded", fn);
 }
}

Usage:

docReady(function() {
 // DOM is loaded and ready for manipulation here
});

If you need full cross browser compatibility (including old versions of IE) and you don't want to wait for window.onload, then you probably should go look at how a framework like jQuery implements its $(document).ready() method. It's fairly involved depending upon the capabilities of the browser.

To give you a little idea what jQuery does (which will work wherever the script tag is placed).

If supported, it tries the standard:

document.addEventListener('DOMContentLoaded', fn, false);

with a fallback to:

window.addEventListener('load', fn, false )

or for older versions of IE, it uses:

document.attachEvent("onreadystatechange", fn);

with a fallback to:

window.attachEvent("onload", fn);

And, there are some work-arounds in the IE code path that I don't quite follow, but it looks like it has something to do with frames.


Here is a 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);

The latest version of the code is shared publicly on GitHub at https://github.com/jfriend00/docReady

Usage:

// pass a function reference
docReady(fn);
// use 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/


Here's a summary of how it works:

  1. Create an IIFE (immediately invoked function expression) so we can have non-public state variables.
  2. Declare a public function docReady(fn, context)
  3. When docReady(fn, context) is called, check if the ready handler has already fired. If so, just schedule the newly added callback to fire right after this thread of JS finishes with setTimeout(fn, 1).
  4. If the ready handler has not already fired, then add this new callback to the list of callbacks to be called later.
  5. Check if the document is already ready. If so, execute all ready handlers.
  6. If we haven't installed event listeners yet to know when the document becomes ready, then install them now.
  7. If document.addEventListener exists, then install event handlers using .addEventListener() for both "DOMContentLoaded" and "load" events. The "load" is a backup event for safety and should not be needed.
  8. If document.addEventListener doesn't exist, then install event handlers using .attachEvent() for "onreadystatechange" and "onload" events.
  9. In the onreadystatechange event, check to see if the document.readyState === "complete" and if so, call a function to fire all the ready handlers.
  10. In all the other event handlers, call a function to fire all the ready handlers.
  11. In the function to call all the ready handlers, check a state variable to see if we've already fired. If we have, do nothing. If we haven't yet been called, then loop through the array of ready functions and call each one in the order they were added. Set a flag to indicate these have all been called so they are never executed more than once.
  12. Clear the function array so any closures they might be using can be freed.

Handlers registered with docReady() are guaranteed to be fired in the order they were registered.

If you call docReady(fn) after the document is already ready, the callback will be scheduled to execute as soon as the current thread of execution completes using setTimeout(fn, 1). This allows the calling code to always assume they are async callbacks that will be called later, even if later is as soon as the current thread of JS finishes and it preserves calling order.

0 votes
Nadira

If you are doing VANILLA plain JavaScript without jQuery, then you must use (Internet Explorer 9 or later):

document.addEventListener("DOMContentLoaded", function(event) {
 // Your code to run since DOM is loaded and ready
});

Above is the equivalent of jQuery .ready:

$(document).ready(function() {
 console.log("Ready!");
});

Which ALSO could be written SHORTHAND like this, which jQuery will run after the ready even occurs.

$(function() {
 console.log("ready!");
});

NOT TO BE CONFUSED with BELOW (which is not meant to be DOM ready):

DO NOT use an IIFE like this that is self executing:

 Example:
(function() {
 // Your page initialization code here - WRONG
 // The DOM will be available here - WRONG
})();

This IIFE will NOT wait for your DOM to load. (I'm even talking about latest version of Chrome browser!)

0 votes
Nadira

There is a standards based replacement,DOMContentLoaded that is supported by over 98% of browsers, though not IE8:

document.addEventListener("DOMContentLoaded", function(event) {
 //do work
});

jQuery's native function is much more complicated than just window.onload, as depicted below.

function bindReady(){
 if ( readyBound ) return;
 readyBound = true;
 // Mozilla, Opera and webkit nightlies currently support this event
 if ( document.addEventListener ) {
 // Use the handy event callback
 document.addEventListener( "DOMContentLoaded", function(){
 document.removeEventListener( "DOMContentLoaded", arguments.callee, false );
 jQuery.ready();
 }, false );
 // If IE event model is used
 } else if ( document.attachEvent ) {
 // ensure firing before onload,
 // maybe late but safe also for iframes
 document.attachEvent("onreadystatechange", function(){
 if ( document.readyState === "complete" ) {
 document.detachEvent( "onreadystatechange", arguments.callee );
 jQuery.ready();
 }
 });
 // If IE and not an iframe
 // continually check to see if the document is ready
 if ( document.documentElement.doScroll && window == window.top ) (function(){
 if ( jQuery.isReady ) return;
 try {
 // If IE is used, use the trick by Diego Perini
 // http://javascript.nwbox.com/IEContentLoaded/
 document.documentElement.doScroll("left");
 } catch( error ) {
 setTimeout( arguments.callee, 0 );
 return;
 }
 // and execute any waiting functions
 jQuery.ready();
 })();
 }
 // A fallback to window.onload, that will always work
 jQuery.event.add( window, "load", jQuery.ready );
}

Use the free JavaScript formatter browser program to manage and tidy your scripts.

0 votes
Nadira

Here is a viable replacement for jQuery ready

function ready(callback){
 // in case the document is already rendered
 if (document.readyState!='loading') callback();
 // modern browsers
 else if (document.addEventListener) document.addEventListener('DOMContentLoaded', callback);
 // IE <= 8
 else document.attachEvent('onreadystatechange', function(){
 if (document.readyState=='complete') callback();
 });
}
ready(function(){
 // do something
});

Category

Follow Us

Stay updated via social channels

Twitter Instagram LinkedIn Instagram
...