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?

score 42 · Accepted Answer · edited Oct 25 '13 at 14:45

42

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 '13 at 14:45

LbISS

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answered Dec 07 '09 at 03:25

o.k.w

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1

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';} – Robert Massaioli Dec 07 '09 at 03:31
@Shhnap: Yes I agree. Just a lazy way out for me :P – o.k.w Dec 07 '09 at 03:34
7

Why bother inserting an `` element? You could just have isPostBack() directly return the value of Page.IsPostBack: `return <%= Page.IsPostBack %>;` – RickNZ Dec 07 '09 at 10:23
@RickNZ: That's a pretty good idea. Post as answer, I'll up-vote it :) – o.k.w Dec 07 '09 at 10:47
Sure: I added it to my earlier answer. – RickNZ Dec 07 '09 at 11:12
6

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. – Shrage Smilowitz Jul 28 '10 at 23:09
This falls down when code-blocks aren't allowed. What would you do in that instance? – Code Maverick Aug 08 '17 at 20:13
Slight variation for culture and language invariance `<%=Page.IsPostBack ? "true" : ""%>` ..... which enables javascript to be more succint too `if (document.getElementById('_ispostback').value) doStuff();` – James Westgate Nov 01 '17 at 10:55

score 22 · Answer 2 · answered Jul 12 '11 at 19:31

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 '11 at 19:31

Daniel

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var offset = document.location.href.indexOf(".aspx"); return document.referrer.substring(0, offset + 5) == document.location.href.substring(0, offset + 5) – Shawn Jan 13 '14 at 23:51
This worked great for me. thanks :) I had a javacript redirect that needed to check for postback. – DigitalRayne Apr 28 '14 at 01:51
What a kludge. Don't use the referrer for logic. – Matti Virkkunen Dec 17 '14 at 18:04

RickNZ · Answer 3 · 2009-12-07T11:11:14.187

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 07 '09 at 11:11

answered Dec 07 '09 at 10:19

RickNZ

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This falls down if you don't have access to server-side code and code blocks aren't allowed. – Code Maverick Aug 08 '17 at 20:11
Oddly, that does not work for me but this does: function IsPostBack() { var ret = '<%= Page.IsPostBack%>' == 'True'; return ret; } – Zeek2 Oct 13 '17 at 12:21

score 6 · Answer 4 · answered Jan 25 '13 at 21:17

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')

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. — Code Maverick, Aug 08 '17 at 20:08

score 5 · Answer 5 · answered Mar 06 '15 at 08:53

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 06 '15 at 08:53

user4640122

51
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1

score 4 · Answer 6 · answered Dec 07 '09 at 06:47

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 07 '09 at 06:47

Brij

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I love your choice of names. – pqsk Jul 23 '13 at 18:28

score 0 · Answer 7 · answered Dec 07 '09 at 03:22

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 07 '09 at 03:22

Robert Massaioli

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score 0 · Answer 8 · answered Feb 14 '17 at 11:31

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
}

score 0 · Answer 9 · answered Jul 06 '18 at 15:56

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;
}

score 0 · Answer 10 · answered Nov 22 '19 at 17:09

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>

score -1 · Answer 11 · answered Jul 25 '13 at 14:07

-1

Here is solution using jQuery:

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

answered Jul 25 '13 at 14:07

Mishami

470
4
4

How to detect/track postback in javascript?

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