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❗ ASP – Error Handling – Manage and Debug Runtime Errors in Classic ASP

🧲 Introduction – Why Error Handling Matters in ASP?

In Classic ASP, runtime errors such as divide-by-zero, null references, or failed database queries can crash your application if left unhandled. Error handling is essential to gracefully detect, log, and respond to errors without exposing sensitive information to end users.

🎯 In this guide, you’ll learn:

How to use On Error Resume Next and the Err object
Ways to log and display custom error messages
Techniques to prevent common ASP errors
Examples for real-world error detection and handling

🚦 Enabling Error Handling with `On Error Resume Next`

<%
On Error Resume Next
' Code that may cause an error
Dim x
x = 1 / 0 ' Division by zero

If Err.Number <> 0 Then
    Response.Write "Error occurred: " & Err.Description
    Err.Clear
End If
%>

🧪 Output:
Error occurred: Division by zero

🛠️ The `Err` Object – Inspecting Errors

Property	Description
`Err.Number`	Error code (0 = no error)
`Err.Description`	Human-readable error message
`Err.Source`	Source of the error
`Err.Clear()`	Resets the error state

📄 Example – Error Logging to File

<%
On Error Resume Next

' Simulate an error
Dim result
result = 10 / 0

If Err.Number <> 0 Then
    Dim fso, file
    Set fso = Server.CreateObject("Scripting.FileSystemObject")
    Set file = fso.OpenTextFile(Server.MapPath("error_log.txt"), 8, True)
    file.WriteLine Now() & " - " & Err.Description
    file.Close
    Set file = Nothing
    Set fso = Nothing
    Err.Clear
End If
%>

🧪 This writes the error to error_log.txt.

❗ Use Case – Preventing Crash on Missing Query Parameter

<%
On Error Resume Next
Dim id
id = Request.QueryString("id")

If IsNumeric(id) Then
    Response.Write "Product ID: " & id
Else
    Response.Write "Invalid product ID."
End If
%>

🔁 Custom Error Messages for Users

<%
On Error Resume Next

' Problematic code
Dim conn
Set conn = Server.CreateObject("ADODB.Connection")
conn.Open "invalid-connection-string"

If Err.Number <> 0 Then
    Response.Write "<p>We’re experiencing technical difficulties. Please try again later.</p>"
    ' Log details, hide from user
    Err.Clear
End If
%>

🧩 Server.GetLastError (ASP 3.0+)

<%
On Error Resume Next
Dim test
test = 5 / 0

Dim lastError
Set lastError = Server.GetLastError()

If Not lastError Is Nothing Then
    Response.Write "Runtime Error: " & lastError.Description
End If
%>

📌 This provides detailed error info without using Err directly.

🧘 Best Practices for Error Handling

✅ Do:

Use On Error Resume Next sparingly and locally
Always check Err.Number after risky operations
Log errors for diagnostics without exposing to users
Clear errors with Err.Clear once handled

❌ Avoid:

Using global On Error Resume Next without checks
Showing raw error details to end users
Ignoring errors during database or file operations

📌 Summary – Recap & Next Steps

Proper error handling in Classic ASP is critical for stability, security, and user experience. By using Err, On Error Resume Next, and logging strategies, you can gracefully manage failures and keep your app running smoothly.

🔍 Key Takeaways:

Use On Error Resume Next + Err to capture runtime errors
Log errors to a file or database for admin visibility
Show custom messages to users without exposing details

⚙️ Real-world Use Cases:

Prevent broken pages from missing form fields
Safely handle file or database connection failures
Log ASP page crashes for developer review

❓ FAQs – ASP Error Handling

❓ What does On Error Resume Next do?
✅ It tells ASP to continue running the script even if an error occurs, allowing you to handle the error manually with Err.

❓ How can I log errors in ASP?
✅ Use FileSystemObject with Server.MapPath() to write logs to a .txt file.

❓ What’s the difference between Err.Number and Server.GetLastError()?
✅ Err.Number is used in all ASP versions, while Server.GetLastError() is a newer method introduced in ASP 3.0+ for detailed error objects.

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