Node.js REPL & Command-Line CLI Flags - Interactive Shell & Modern Runtime Switches

The Node.js REPL (Read-Eval-Print Loop) is an indispensable scratchpad for testing JavaScript syntax, inspecting core APIs, and experimenting with asynchronous code. In addition, mastering Node's native command-line interface (CLI) flags empowers you to run, watch, and debug applications like an industry professional.

Starting and Using the Node.js REPL

To enter the REPL, simply type node in your terminal with no arguments:

$ node
Welcome to Node.js v22.11.0.
Type ".help" for more information.
> 2 + 5 * 10
52
> const os = require('os');
undefined
> os.totalmem() / (1024 * 1024 * 1024);
15.892765045166016 // Gigabytes of RAM on this machine
> 

Key REPL Commands and Shortcuts

Essential Modern Node.js CLI Flags (Production & Dev)

Modern versions of Node.js include powerful built-in flags that eliminate the need for third-party utilities:

1. --watch (Native File Watcher)

Replaces nodemon. Node automatically restarts the process whenever source files are modified:

node --watch server.js

2. --env-file=.env (Native Environment Variables)

Replaces the dotenv package. Automatically loads key-value pairs from your .env file directly into process.env:

node --env-file=.env index.js

3. --inspect (Chrome DevTools Debugger)

Opens a debugging WebSocket port (default 127.0.0.1:9229). Open Google Chrome and visit chrome://inspect to set visual breakpoints, step through code line-by-line, and inspect the memory heap:

node --inspect server.js
# Or break at the very first line before running:
node --inspect-brk server.js

4. --trace-warnings (Debugging Deprecations)

Prints full stack traces for runtime warnings and deprecation notices, making it easy to identify the exact file and line causing an issue:

node --trace-warnings index.js