How to execute javascript onLoad only once in an ASP.NET page

If you are working on a ASP.NET Master/Content Page scenario and have declared a javascript onload, then the script gets executed every on every postback.

However if you would like to execute it only once during the first postback, then here's how to do so:
Copy this code in the <head> of your MasterPage



<script type="text/javascript">


function invokeMeMaster() {


var chkPostBack = '<%= Page.IsPostBack ? "true" : "false" %>';


if (chkPostBack == 'false') {


alert('Only the first time');


}

}



window.onload = function() { invokeMeMaster(); };


</script>



The code determines if there is a postback in Javascript and then executes code based on the result. Hence the alert popup is shown only once.

You can several other tips related to calling JavaScript in Master/Content Pages over here:

Calling JavaScript from ASP.NET Master Page and Content Pages - Part I

Calling JavaScript from ASP.NET Master Page and Content Pages - Part II

4 comments:

  1. darenkovFebruary 19, 2009 at 3:56 PM

    This throws and error. BC30037: Character is not valid. I think it has something to do with the angle brackets.

    ReplyDelete
  2. Suprotim AgarwalFebruary 20, 2009 at 3:45 AM

    Please post the entire code over here and I will have a look at it.

    ReplyDelete
  3. AnonymousSeptember 29, 2009 at 7:08 AM

    How in PHP?

    ReplyDelete
  4. UnknownAugust 16, 2012 at 1:25 PM

    This site prevents the posting of any tags other than bold, italics or link.
    But if you create a simple aspx page and add your suggested javascript you will see that it returns a
    "Compiler Error Message: BC30037: Character is not valid."
    Put simply the .net 2 server doesn't like
    var chkPostBack = '<%= Page.IsPostBack ? "true" : "false" %>';

    ReplyDelete