When using ASP, it's important to understand that ASP code exists only on the server. Code surrounded by <% and %> delimiters gets processed completely on the server. The client never accesses this code. If you've created web pages before, you're likely familiar with client-side scripting, which works differently.
Client-side scripting consists of programmatic code in an HTML file that runs in the browser. This code uses the <script> HTML tag. JavaScript is the most common client-side scripting language because Netscape Navigator only supports JavaScript for client-side operations.
Here's a simple example of a static HTML page with client-side scripting:
<HTML>
<HEAD>
<SCRIPT LANGUAGE="JavaScript">
<!--
alert("Hello world");
//-->
</SCRIPT>
</HEAD>
<BODY>
Welcome to my web page!
</BODY>
</HTML>
When you save the code above as clientside.htm, the entire file gets sent to the browser when the client requests the page. The browser displays a message box when it reaches the alert method during rendering. The web server treats client-side scripting as regular HTML code.
You can include client-side scripting code in an ASP page because the web server sees client-side scripting as plain HTML. The server passes it through unchanged to the browser.
Server-side scripts are scripts that execute on the web server. These scripts get processed completely, and their output is sent to the client. The client never sees the actual script code.
<%@ Language="VBScript" %>
<HTML>
<HEAD>
<TITLE>Server-Side Example</TITLE>
</HEAD>
<BODY>
<%
' This code runs on the server
Dim userName
userName = Request.Form("name")
If userName <> "" Then
Response.Write "<h1>Welcome, " & userName & "!</h1>"
Else
Response.Write "<h1>Welcome, Guest!</h1>"
End If
' Server generates timestamp
Response.Write "<p>Page generated at: " & Now() & "</p>"
%>
</BODY>
</HTML>The web server doesn't process client-side scripts at all. Only the client processes them. It's the client's responsibility to execute all client-side scripts:
<SCRIPT LANGUAGE="JavaScript">
function validateForm() {
var email = document.forms["contactForm"]["email"].value;
if (email == "") {
alert("Email must be filled out");
return false;
}
// Check email format
var emailPattern = /^[^\s@]+@[^\s@]+\.[^\s@]+$/;
if (!emailPattern.test(email)) {
alert("Please enter a valid email address");
return false;
}
return true;
}
</SCRIPT>
<FORM name="contactForm" onSubmit="return validateForm()">
Email: <INPUT type="text" name="email">
<INPUT type="submit" value="Submit">
</FORM>Server-side scripts get processed completely on the web server. The client doesn't receive any code from server-side scripts—just the output. Client-side scripts and server-side scripts can't interact directly because they execute at different times and places:
<%
' This runs on the server only
Dim conn, rs, sql
' Connect to database
Set conn = Server.CreateObject("ADODB.Connection")
conn.Open "DSN=MyDatabase"
' Execute query
sql = "SELECT ProductName, Price FROM Products WHERE Category='Electronics'"
Set rs = conn.Execute(sql)
' Generate HTML from results
Response.Write "<table border='1'>"
Response.Write "<tr><th>Product</th><th>Price</th></tr>"
While Not rs.EOF
Response.Write "<tr>"
Response.Write "<td>" & rs("ProductName") & "</td>"
Response.Write "<td>$" & rs("Price") & "</td>"
Response.Write "</tr>"
rs.MoveNext
Wend
Response.Write "</table>"
rs.Close
conn.Close
%>You can use both server-side and client-side scripting together effectively. The server-side code runs first and can generate client-side code dynamically:
<%@ Language="VBScript" %>
<HTML>
<HEAD>
<SCRIPT LANGUAGE="JavaScript">
function showWelcome() {
var userName = "<%= Session("UserName") %>";
alert("Welcome back, " + userName + "!");
}
</SCRIPT>
</HEAD>
<BODY onLoad="showWelcome()">
<%
' Server-side: Check if user is logged in
If Session("UserName") <> "" Then
Response.Write "<h1>Dashboard</h1>"
Response.Write "<p>Last login: " & Session("LastLogin") & "</p>"
Else
Response.Redirect "login.asp"
End If
%>
</BODY>
</HTML>Understanding that client-side and server-side scripts are two completely separate entities is essential. They execute at different times, in different locations, and serve different purposes. Client-side scripts handle user interactions and provide immediate feedback. Server-side scripts handle business logic, database operations, and generate dynamic content. Use each where it makes the most sense for your application.
No, client-side and server-side scripts cannot directly interact because they execute at different times and locations. Server-side scripts run first on the server and finish completely before any client-side scripts execute in the browser. However, server-side code can generate client-side code dynamically, allowing you to pass data from the server to client-side scripts.
Use server-side scripting for database operations, sensitive business logic, user authentication, and generating dynamic content. Use client-side scripting for form validation, user interface interactions, animations, and immediate feedback without server round-trips. Often you'll use both together for the best user experience.
The web server processes all ASP code completely before sending anything to the browser. Only the output generated by your ASP code reaches the client, never the code itself. This protects your source code and ensures browser compatibility since the browser only receives standard HTML. It's like the server showing you the finished meal instead of the recipe.