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_doPostBack in JavaScript

_doPostBack sounds familiar to most of the people who are having hands on experience on .Net web application. Most of you have already dissected each and everything about this function. But there are some people who are still wanted to know about it. So here we go.

_doPostBack is a JavaScript function that controls in the webform uses to trigger a postback.Button and ImageButton are two exceptions which doesn’t use this function to trigger the postback.

Let us do a simple exercise to know about this method

    • First of all open visual studio and create a web application DissectDoPostBack(you can give any name to your web application).

    • On the default.aspx page add a button control.

    • Run the application.

    • You can see a button which have added in the web form at design time.

    • Right click on the web browser.

    • Click View Source.

Picture showing the rendered html of the web page without _dopostback function

Click to Enlarge

As you can see in the above figure that there is no _doPostBack function on the page because button doesn’t use _doPostBack to trigger a postback.

• Now add a textbox in the form.

Again run the application.

• Right click on the browser window and click View Source

Click to Enlarge

Surprised!! Still there is no _doPostBack function in the page, but as stated earlier that except button and ImageButton all other controls uses this method to trigger a postback.Lets move forward

    • Now add an attribute AutoPostBack = "true" in the textbox.

    • Again run the application.

    • Click on the textbox and type something in it. You can observe the submission of page validates AutoPostBack = "true" Fair enough.

    • Now again right click on the browser and click View Source

Picture showing the rendered html of the web page with _dopostback function

Click to Enlarge

Check the onChange event of textbox. It means whenever there is a change in the text of the textbox, the textbox uses _doPostBack method to trigger a postback.

Now let us delve more into this method._doPostBack takes 2 arguments eventTarget and eventArgument.The first parameter is the ID of the control that triggers the postback and second parameter contains any additional data associated with the control.

The value of both the controls is stored in _EVENTTARGET and _EVENTARGUMENThidden fields

Now let us call _doPostBack explicitly on button click.

• On the head section of the page add a JavaScript function ClickButton

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

    return false;
  } </script>

• On the page load add the following code

Button1.Attributes.Add("onclick", "return ClickButton();");

• Run the application

When you click on the button you will observe that page is not submitting because in the JavaScript method ClickButton we are returning false.

• Now in the ClickButton • method add following

_doPostBack("TextBox1", "Test");

So our function becomes

<script language = "javascript"
type = "text/javascript">
  function ClickButton() {
    _doPostBack("TextBox1", "Test");
    return false;
  } </script>

• Now again run the application.

• Click on the button.

Page is submitting successfully. In the above code we are using textbox to trigger a postback.

I hope you like this tutorial. If you have any queries please send me by clicking the link below.

Posted By -	Karan Gupta

Posted On -	Friday, May 21, 2010

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