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

About The Author

Suprotim Agarwal, Developer Technologies MVP (Microsoft Most Valuable Professional) is the founder and contributor for DevCurry, DotNetCurry and SQLServerCurry. He is the Chief Editor of a Developer Magazine called DNC Magazine. He has also authored two Books - 51 Recipes using jQuery with ASP.NET Controls. and The Absolutely Awesome jQuery CookBook.

Follow him on twitter @suprotimagarwal.

4 comments:

darenkov said...: This throws and error. BC30037: Character is not valid. I think it has something to do with the angle brackets.; February 19, 2009 at 3:56 PM
Suprotim Agarwal said...: Please post the entire code over here and I will have a look at it.; February 20, 2009 at 3:45 AM
Anonymous said...: How in PHP?; September 29, 2009 at 7:08 AM
Unknown said...: 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" %>';; August 16, 2012 at 1:25 PM