$(document).ready(), pageLoad() and Sys.Application.add_init()

When I have basic understanding of AJAX and jQuery, I thought that $(document).ready(), pageLoad() and Sys.Application.add_init() are equivalent and they does the same job. In most of the samples they seems to behave in same way and I wondered that why there are too many ways to do the same thing. So I did bit research on this and I am sharing that with the developer community.

jQuery's $(document).ready()
This function is called when DOM is ready. Generally if browser supports DOMContentLoaded event, $(document).ready() will fire after this event. If browser do not support DOMContentLoaded, then when document achieve readyState, then $(document).ready() will be fired. If above two events are not available in Brower's event model then window.onload event is used to fire $(document).ready() event.

If you are using jQuery, then my No. 1 recommendation is this function as this will work on
all browsers. Here is a sample how you can use this function:-

$(document).ready(function () {
//Your code here
});

Application.Init
This event is fired only once when page is loaded first time. Remember, that if you are using update panel then this method will not be called whenever update panel refresh your page. So, this is best event when you want initialization which should be done only once and not after every update panel refresh.

<script type="text/javascript">
    Sys.Application.add_init(function () {
        // Your code here
    }); 
script>

ASP.NET AJAX's pageLoad() Method
As we all know java scripts runs as a single thread, pageLoad method uses a trick, it calls a setTimeout() function of JavaScript with 0. So, whenever DOM is ready and JavaScript execution is started, you  pageLoad() will be triggered.

Now, use this method when you want to call your code every time when your update panel gets refresh.

<script type="text/javascript">
    function pageLoad() {
        // Your code here 
    } 
script>

Note that to use AJAX methods you need to place ScriptManager object on  your page other wise they won't work.

Feel free to comment, Happy Coding !!!