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_Dopostback In Javascript

In this article, we will discuss _doPostBack in JavaScript.

Purpose

The _doPostBack is a JavaScript function that triggers a post back.

Example

Let us work on an example. Create a web application using Visual Studio. In default.aspx, add a button control. Run the application

Right-click on the web browser. Click View Source.

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

The above picture shows no _doPostBack function on the page. The reason is the button and ImageButton does not use _doPostBack to trigger a postback. Add a textbox in the form with AutoPostBack equals to true. Right-click on the browser window and click View Source.

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

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

More Info About _doPostBack

The _doPostBack takes 2 arguments: -

eventTarget → ID of the control that triggers the postback
eventArgument → contains any additional data associated with the control

The value is stored in _EVENTTARGET and _EVENTARGUMENT hidden fields.

Calling The _DoPostBack Function On Button Click

Write the following script in the head section.

<script 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 the page is not submitting because in the JavaScript method ClickButton we are returning false.

Modify the code and add the following to the ClickButton method.

_doPostBack("TextBox1", "Test");

So, our function becomes.

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

Try It

Run the application and click on the button. The page is submitted successfully. In the above code, we use a textbox to trigger a postback.

Posted By -	Karan Gupta

Posted On -	Friday, May 21, 2010

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