How to call javascript function in ClassLibrary project

How can I call a javascript function from my CS page in c#?

A class-library is basically a DLL that mostly encapsulates code and you can reference to from other class-libraries(DLLs) and use - The client code in your case would probably be an ASP.NET application DLL, I assume.

You can always call a JavasScript function from a C# / Class-Library using the RegisterStartupScript method:

Page.ClientScript.RegisterStartupScript(this.GetType(),"CallMyFunction","MyFunction()",true);

Just take on account that this code should be run in the context of a ASP.NET Page class in the event of the page is getting loaded (Page_Load).

In the case of a Class-Library, just make sure to have the right reference to it in your project (Right-Click on your project → Select Add Reference → Select and add your class-library).

Example code:

public void Page_Load(Object sender, EventArgs e)
{
    String csname1 = "PopupScript";
    String csname2 = "ButtonClickScript";
    Type cstype = this.GetType();

    ClientScriptManager cs = Page.ClientScript;

    if (!cs.IsStartupScriptRegistered(cstype, csname1))
    {
      String cstext1 = "alert('Hello World');";
      cs.RegisterStartupScript(cstype, csname1, cstext1, true);
    }
}

You can pass the Page reference to class Library and execute the JavaScript function inside the class library.

MyClassLib lib = new MyClassLib();
lib.RunJs(this.Page);

// In Class Lib 

Public Class MyClassLib 
{
  public void RunJs(System.Web.UI.Page page)
  {
      ClientScriptManager js = Page.ClientScript;
      ......
  }
}

I have not tried this please have a try on this.

You can call JavaScript Function from code behind using ScriptManager.RegisterStartupScript.

like:

JS Code:

function TestFunction() { 
       Alert("Test Function!!") 
       } 

Code Behind:

Page.ClientScript.RegisterStartupScript(this.GetType(), "alert", "TestFunction();", true);