We normally add JavaScript functions to our webpage using <Script> tag. There are situations where we need to add the scripts dynamically from the codebehind class. In .NetFramework 1.x version, there is no class that helps us to handle this situation effectively. This drawback was addressed .NetFramework 2.0 by introducing a new class called ClientScriptManager.  This class can be used to manage and add script blocks to the asp.net page from codebehind class.

Ø       ClientScript property of the Page object will give us an instance of ClientScriptManager object. We can add the scripts dynamically through this instance which will be then injected in the HTML output.

Ø       This class uniquely identifies scripts by a key String and a Type. Scripts with the same key and type are considered duplicates and hence similar scripts are avoided. This will avoid the confusion caused when we are adding scripts from usercontrols. For example, the method IsClientScriptBlockRegistered() can be used for checking duplicate script registered for RegisterClientScriptBlock() method.

ClientScriptManager class has a set of useful methods which we can use to inject the JavaScript functions in the HTML output. We can choose to use these methods to accomplish our requirements depending on the situation.

Moving forward, we will discuss the usages of below 3 different methods to inject javascript from codebehind file,

1.      RegisterClientScriptBlock() Methods.

2.      RegisterStartupScript() Methods.

3.      RegisterOnSubmitStatement() Methods.

Page.RegisterStartUpScript() and the Page.RegisterClientScriptBlock() methods in .Netframework 1.x are now considered obsolete. These 2 methods are now packed with ClientScriptManaget class. The RegisterClientScriptBlock() method allows you to place a JavaScript function at the top of the page and gets executed on startup of the page i.e. when loading the page in the browser. There is a additional method called IsClientScriptBlockRegistered() in ClientScriptManager which will return true if a script block is already registered with  the same key, hence we can prevent the duplicate script registration.

There are 2 overloads for this method,

Placing this code on page load or a button click makes the script to fire on the start up of subsequent postback.

As I said earlier, these methods will make the script block to execute on the startup, thus we can see the alert box before the controls are actually rendered.

In this section, we will see the usage ClientScriptManager.RegisterStartupScript() method of ClientScriptManager class. Both, RegisterStartupScript() methods and RegisterClientScriptBlock() methods will inject Jscript code that will fire during start up of subsequent postback. But the real difference is the former methods will inject the script after the form open tag but before page controls and the RegisterStartupScript() methods will inject the scripts after page controls but before form close tag. This indicates that injecting script using RegisterClientScriptBlock() method it is not possible to access page controls in the script block while using RegisterStartupScript() method we can.

The below markups will show a part an html output given by the asp.net page when executing these RegisterClientScriptBlock and RegisterStartupScript methods.

In the above html snippet, we can see the script embedded before the page controls but after form open tag.

 In the above html snippet, we can see the script embedded after the page controls but before form close tag thus making the script able to access the page controls as I said earlier.

There are 2 overloads for this method,

Placing this code on page load or a button click makes the script to fire on the start up of subsequent postback. This method is also has a method called IsStartupScriptRegistered() like RegisterClientScriptBlock() methods which will check for script duplications.

When the above code is executed we will get an output like,

Here, the script block will get executed after the controls in the page are rendered and the controls in the page will be visible to the script as opposed to RegisterClientScriptBlock() method, refer the above figure. Thus we can access the page controls from the script block when using RegisterStartupScript() method.

This section will discuss the implementation of ClientScriptManager.RegisterOnSubmitStatement() of ClientScriptManager class. Sometimes, we will require getting confirmation from the user before submitting the form to server. For example, in an input form we would like to get the confirmation from the users before storing it to the database whose validity cannot be determined through validation functions. This method can be used in such situations to provide a confirmation dialogs. This method registers scripts which will be executed at the time of submit click of a page.

Placing this code on page load makes the script to fire on every submit click of the webform.

Consider the below code,

Executing the above code will bring a webform, Clicking Submit will bring a confirm box like the below figure,

Clicking Cancel will not execute the submit click event, where clicking OK will execute the event and output will be like,