npx asp-classic-emulator

What is this?

ASP Classic never died — it just got stranded on Windows. Millions of lines of VBScript are still running in production, and maintaining them means either keeping a Windows Server around or setting up a VM every time someone needs to touch the code.

asp-classic-emulator fixes that. Drop it into any folder with .asp files, run one command, and get a fully working HTTP server that interprets VBScript — on any POSIX system.

your-project/
├── index.asp
├── login.asp
└── includes/
    └── conn.asp

cd your-project
npx asp-classic-emulator
# → Serving on http://localhost:3000

Note: This project is in active development. See the Implemented and Planned sections below for an accurate picture of what works today.

Quick start

Requirements

• Node.js ≥ 18
• macOS, Linux, or BSD

Run without installing

npx asp-classic-emulator

Install globally

npm install -g asp-classic-emulator
asp-classic-emulator

Install as a project dependency

npm install asp-classic-emulator

import { createServer } from 'asp-classic-emulator'

const server = createServer({
  root: './www',
  port: 3000,
  mode: 'dev',
})

await server.start()

Implemented (v0.1.0)

HTTP server

• Serves .asp files through the VBScript interpreter
• Serves static files (.css, .js, .png, .jpg, .gif, .svg, .ico) with correct Content-Type
• Parses QueryString and Form (URL-encoded and multipart)
• Request log: GET /index.asp 200 11ms
• Dev error page — file name, line, column, source excerpt, stack trace
• Prod error page — classic IIS-style (Microsoft VBScript runtime error '800a000d')

VBScript — Phase 1

ASP objects

Type system

• Full Variant type with subtypes: Empty, Null, Boolean, Integer, Long, Double, String, Object
• Implicit coercion works as in IIS — warning printed to terminal
• --strict flag turns coercions into runtime errors

#include

• <!--#include file="..." --> and <!--#include virtual="..." -->
• Circular reference detection (throws with full cycle path)

CLI flags (wired)

Config file

Create asp-classic-emulator.config.json in your project root. CLI flags override config values.

{
  "port": 3000,
  "root": "./www",
  "mode": "dev",
  "strict": false,
  "logLevel": "info"
}

Planned

The following features are on the roadmap but not yet implemented.

VBScript — Phase 1 (remaining)

VBScript — Phase 2

ASP objects (remaining)

Database support

Other

Type system

VBScript's Variant type is fully emulated. Every variable can hold any subtype and changes at runtime — just like the original.

Dim x
x = 42      ' Long
x = "hello" ' String
x = True    ' Boolean
x = Null    ' Null (contagious in expressions)
x = Empty   ' Empty (zero in numeric context)

Implicit coercion

By default, implicit coercions work exactly as in IIS, and a warning is printed to the terminal:

[WARN] Implicit coercion in /dashboard.asp:12 — String "42" → Number
       Use CInt() or CDbl() for explicit conversion. Run with --strict to throw instead.

Use --strict to turn all implicit coercions into runtime errors:

asp-classic-emulator --strict

Architecture

HTTP Request
     │
     ▼
HTTP Server (node:http)
     │
     ▼
Request Parser  ──  QueryString, Form, Headers
     │
     ▼
ASP Pre-processor  ──  resolve #include, split HTML / <% %>
     │
     ▼
Lexer (Chevrotain)  ──  text → typed token stream
     │
     ▼
Parser (Chevrotain)  ──  tokens → AST with line/column info
     │
     ▼
Interpreter  ──  walks AST, injects ASP objects, executes VBScript
     │
     ▼
HTTP Response  ──  flush HTML buffer to client

Why a hand-written parser?

No usable VBScript parser exists in the Node.js ecosystem. The few packages that exist are abandoned, incomplete, or cover less than 60% of the grammar.

Chevrotain was chosen because:

• TypeScript native — no separate grammar file, no code generation step
• Incrementally extensible — adding Select Case or With is a new method, nothing existing breaks
• Built-in error recovery — precise line and column in every error message
• Production-grade — used by SAP, MongoDB, and others

Contributing

Contributions are welcome, especially:

• VBScript built-in functions (Split, Join, Replace, Mid, DateAdd, etc.)
• ReDim, Exit, and Function return value (Phase 1 remaining items)
• Additional Response and Request members
• Real-world ASP fixture files for the test suite (anonymized is fine)
• Bug reports with .asp snippets that don't behave as expected

git clone https://github.com/MarceloDelgadoDev/asp-classic-emulator
cd asp-classic-emulator
npm install
npm run dev
npm test

License

MIT — see LICENSE for details.