How to detect/track postback in javascript?

Question

How to detect/track/check postback in javascript(e.g in asp.net Page.isPostBack())? Any suggestion?

LbISS · Accepted Answer · 2013-10-25 14:45:28Z

43

ASPX:

<input type="hidden" id="_ispostback" value="<%=Page.IsPostBack.ToString()%>" />

Client-side Script:

function isPostBack() { //function to check if page is a postback-ed one
  return document.getElementById('_ispostback').value == 'True';
}

PS: I have not tested it but I've done somthing similar before and it works.

edited Oct 25, 2013 at 14:45

LbISS

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answered Dec 7, 2009 at 3:25

o.k.w

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8 Comments

Robert Massaioli Over a year ago

As a matter or preference I add runat="server" to the hidden input and change it's value in the codebehind file but this works just fine. And I would recommend jQuery to do the javascript because that would become: function isPostBack() {return $("#_ispostback").val() == 'true';}

o.k.w Over a year ago

@Shhnap: Yes I agree. Just a lazy way out for me :P

RickNZ Over a year ago

Why bother inserting an <input> element? You could just have isPostBack() directly return the value of Page.IsPostBack: return <%= Page.IsPostBack %>;

o.k.w Over a year ago

@RickNZ: That's a pretty good idea. Post as answer, I'll up-vote it :)

Shrage Smilowitz Over a year ago

There is a reason why a hidden field is better, just using a javascript function will only load this value on actual post back, but for asynch postback it will not, using an input field within a an update panel that is set to updatemode=always, will make this work even on asynch postbacks.

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Daniel · Accepted Answer · 2011-07-12 19:31:56Z

22

In some cases, you may want to check for postback without any server-side code. For example, in SharePoint, you cannot have code blocks in SharePoint Designer pages, so you can't use any solution that requires <%=something %>. Here is an alternative that involves no server-side code:

<script type="text/javascript">
 function isPostBack()
 {

  return document.referrer.indexOf(document.location.href) > -1;
 }

 if (isPostBack()){
document.write('<span style="color:red;">Your search returned no results.</span><br/>');
 }
 </script>

One caveat (or feature, depending on how you look at it), this will detect not just postbacks, but any instance where the page links to itself.

answered Jul 12, 2011 at 19:31

Daniel

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3 Comments

Shawn Over a year ago

var offset = document.location.href.indexOf(".aspx"); return document.referrer.substring(0, offset + 5) == document.location.href.substring(0, offset + 5)

DigitalRayne Over a year ago

This worked great for me. thanks :) I had a javacript redirect that needed to check for postback.

Matti Virkkunen Over a year ago

What a kludge. Don't use the referrer for logic.

RickNZ · Accepted Answer · 2009-12-07 11:11:14Z

9

If you want to check whether the current page will be a postback if the user clicks on a submit button, you can check for the presence of ViewState:

<input type="hidden" name="__VIEWSTATE" id="__VIEWSTATE" value="xxxxx" />

You can use something like document.getElementById("__VIEWSTATE") or the jQuery equivalent.

However, if you want to see whether the current page was generated in response to a postback, then you need to insert that data into the page on the server side first.

For example:

function isPostBack() {
  return <%= Page.IsPostBack %>;
}

edited Dec 7, 2009 at 11:11

answered Dec 7, 2009 at 10:19

RickNZ

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2 Comments

Code Maverick Over a year ago

This falls down if you don't have access to server-side code and code blocks aren't allowed.

Zeek2 Over a year ago

Oddly, that does not work for me but this does: function IsPostBack() { var ret = '<%= Page.IsPostBack%>' == 'True'; return ret; }

zzzzBov · Accepted Answer · 2013-01-25 21:17:03Z

6

As JavaScript shouldn't be written with server-side code, and injecting new elements into the page seems like overkill, it seems to me that the simplest solution is to add [datat-*] attributes to the <head> element:

_{In Page_Load:}

Page.Header.Attributes["data-is-postback"] IsPostBack ? "true" : "false";

This can then be accessed as:

_jQuery:

$('head').data('isPostback');

_{Vanilla JS:}

document.head.getAttribute('data-is-postback') === 'true';

Of course, if you treat the [data-is-postback] attribute as a boolean attribute, you could alternatively use:

_{In Page_Load:}

if (IsPostBack)
{
    Page.Header.Attributes.Add("data-is-postback", "");
}
else
{
    Page.Header.Attributes.Remove("data-is-postback");
}

_jQuery:

$('head').is('[data-is-postback]');

_{Vanilla JS:}

document.head.hasAttribute('data-is-postback')

answered Jan 25, 2013 at 21:17

zzzzBov

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1 Comment

Code Maverick Over a year ago

While this is great if you access to server-side code, it falls down if you don't. Some software doesn't even allow code blocks, so a pure JavaScript solution would be needed.

user4640122 · Accepted Answer · 2015-03-06 08:53:18Z

5

I have a solution that worked for me.

// Postback catch
var prm = Sys.WebForms.PageRequestManager.getInstance();
prm.add_endRequest(function (s, e) {
    alert("post back");
});

answered Mar 6, 2015 at 8:53

user4640122

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Comments

Brij · Accepted Answer · 2009-12-07 06:47:06Z

4

See following:

<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>

answered Dec 7, 2009 at 6:47

Brij

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1 Comment

pqsk Over a year ago

I love your choice of names.

Robert Massaioli · Accepted Answer · 2009-12-07 03:22:40Z

0

You can only keep track of the postback if you are using AJAX requests or have a hidden field of some sort that the javascript reads on page load. Otherwise the page is regenerated and all POST data is lost; as you would expect and hope.

answered Dec 7, 2009 at 3:22

Robert Massaioli

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Comments

Lequoa · Accepted Answer · 2017-02-14 11:31:37Z

0

on Page_Load on your server-side : The following uses an overload of RegisterClientScriptBlock() that will surround our string with the needed script tags

Server-Side

if (Page.IsPostBack){
   ClientScript.RegisterClientScriptBlock(GetType(),
            "IsPostBack", "var isPostBack = true;", true);
}

Then in your script which runs for the onLoad, check for the existence of that variable.

if (isPostBack){
   //do something here
}

answered Feb 14, 2017 at 11:31

Lequoa

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Comments

Olymp Medco · Accepted Answer · 2018-07-06 15:56:11Z

0

This should work for ASP.Net pages without relying on a backend supplied variable/control:

function isPostBack(frmID) {
    var eventtarget = "";
    var eventargument = "";

    if (!!frmID) {
        if (document.forms.length == 0) return false;

        sForm = document.forms[0];
    }
    else {
        sForm = document.getElementById(frmID);

        if (!sForm) return false;
    }

    if (sForm.__EVENTTARGET) eventtarget = sForm.__EVENTTARGET.value;
    else return false;

    if (sForm.__EVENTARGUMENT) eventargument = sForm.__EVENTARGUMENT.value;
    else return false;

    if (eventtarget != "" || eventargument != "") return true;
    else return false;
}

answered Jul 6, 2018 at 15:56

Olymp Medco

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Comments

Sir Asks a Lot · Accepted Answer · 2019-11-22 17:09:14Z

0

This is a simple JS way to determine the status of IsPostBack that I just got working in the Body of my ASPX page; needed to cause a PostBack during PageLoad for a project.

    <script type="text/javascript">
        if ('False' === '<%= Page.IsPostBack.ToString()%>') 
        {
            __doPostBack();
        }
    </script>

answered Nov 22, 2019 at 17:09

Sir Asks a Lot

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Comments

Mishami · Accepted Answer · 2013-07-25 14:07:15Z

-1

Here is solution using jQuery:

$("a[href^='javascript:__doPostBack']").click(function () {
    alert('ok');
});

answered Jul 25, 2013 at 14:07

Mishami

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