Simple JS REPL

A secure JavaScript REPL (Read-Eval-Print-Loop) for executing code snippets with comprehensive error handling, memory management, and output formatting.

Features

• Secure Execution Environment: Runs JavaScript code in a sandboxed VM context
• Memory Limit Protection: Prevents memory-intensive operations from crashing the server
• Timeout Handling: Automatically terminates long-running operations
• Comprehensive Error Reporting: Detailed error messages with stack traces
• Console Output Capture: Captures and formats all console methods (log, error, warn, etc.)
• Support for Async/Await: Execute asynchronous code with top-level await
• Node.js Module Access: Import and use Node.js built-in modules securely
• 109 Test Cases: Extensive testing for all features and edge cases

Quick Start with npx

Run the REPL without installing it:

npx simple-js-repl

This will start an interactive REPL where you can enter JavaScript code:

> return 2 + 2
--- No Console Output ---

--- Result ---
4
------------

You can also execute code directly:

npx simple-js-repl "console.log('Hello world'); return 42"

Or pipe code into it:

echo "return new Date().toISOString()" | npx simple-js-repl

Installation

If you want to install the package globally:

npm install -g simple-js-repl

After installation, you can use it as:

js-repl

CLI Usage

Simple JavaScript REPL - Secure code execution environment

Usage:
  $ npx simple-js-repl
  $ npx simple-js-repl "console.log('Hello, World!')"
  $ echo "return 2 + 2" | npx simple-js-repl

Options:
  --help, -h    Show this help message
  --version, -v Show version number

API Usage

The REPL server exposes a single MCP tool called execute that accepts JavaScript code as input and returns the execution result along with any console output.

Example MCP request:

{
  "jsonrpc": "2.0",
  "id": "1",
  "method": "tool",
  "params": {
    "name": "execute",
    "arguments": {
      "code": "return 2 + 2;"
    }
  }
}

Example response:

{
  "jsonrpc": "2.0",
  "id": "1",
  "result": {
    "content": [
      {
        "type": "text",
        "text": "--- No Console Output ---\n\n--- Result ---\n4\n------------"
      }
    ],
    "isError": false
  }
}

Testing

The package includes comprehensive test suites:

# Run all tests
npm test

Security

The REPL runs code in a secure sandboxed environment with:

• Restricted access to sensitive modules (fs, child_process, etc.)
• Memory usage monitoring and limits
• Execution timeouts
• Proper error isolation and reporting

License

MIT