Register the JavaScript function at Code-Behind

Method 1: Use RegisterClientScriptBlock

if (!this.IsClientScriptBlockRegistered("clientScript"))
{
String scriptString = "<script language=\"JavaScript\"> ";
scriptString += "function YourJavascriptFunction() {";
scriptString += "} ";
scriptString += "";
this.RegisterClientScriptBlock("clientScript", scriptString);
}

Method 2: Use RegisterStartupScript

this.Page.ClientScript.RegisterStartupScript(this.GetType(),"alert","<script>alert(‘hello’);</script>");

Method 3: Use Literal

private void Button2_Click(object sender, System.EventArgs e)
{
string str;
str="<script language='JavaScript'>";
str+="selectRange()";
str+="<script>";
Literal1.Text=str;
}