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The REPL Module in Node.js

The repl module in Node.js provides a Read-Eval-Print-Loop (REPL) environment that can be used as a standalone program or embedded within applications. It allows developers to interactively execute JavaScript code, inspect results, and experiment with Node.js features. REPL supports input completion, history, multi-line editing, ANSI-styled output, error recovery, and custom evaluation functions.

repl.REPLServerSpecial commands (.break, .clear, .exit, .help, .save, .load, .editor)Events: exit, resetContext and global variables

~2 min read • Updated Dec 30, 2025

1. Introduction


The repl module is loaded with require('node:repl'). It exports the REPLServer class, which reads user input, evaluates it, and prints the result. Input and output can be connected to stdin and stdout or other streams.


2. Features


  • Automatic input completion.
  • Emacs-style line editing.
  • Multi-line input support.
  • ZSH-like reverse-i-search and substring-based history search.
  • ANSI-styled output.
  • Persistent session history.

3. Special Commands


  • .break: Abort multi-line input.
  • .clear: Reset REPL context.
  • .exit: Exit the REPL.
  • .help: Show available commands.
  • .save: Save session to a file.
  • .load: Load a file into the session.
  • .editor: Enter editor mode.

4. Default Evaluation


By default, REPL evaluates JavaScript expressions and provides access to Node.js core modules. Variables declared are global unless scoped within blocks or functions.


5. Context and Variables


Developers can expose variables to REPL by assigning them to the context object. Properties can be made read-only using Object.defineProperty().


6. Error Handling


  • Uncaught exceptions are managed using the domain module.
  • The special variable _ stores the last evaluated result.
  • _error stores the last error.

7. Await Support


Top-level await is supported in REPL, allowing asynchronous code execution directly. However, redeclaring constants after using await may cause errors.


8. Custom Commands


New commands can be defined using defineCommand():


replServer.defineCommand('sayhello', {
  help: 'Say hello',
  action(name) {
    console.log(`Hello, ${name}!`);
  },
});

9. Events


  • exit: Triggered when REPL exits.
  • reset: Triggered when REPL context is cleared.

10. History and Environment Variables


REPL saves input history to .node_repl_history. Environment variables like NODE_REPL_HISTORY and NODE_REPL_HISTORY_SIZE control persistence and size.


11. Advanced Examples


  • Running REPL over TCP or Unix sockets.
  • Embedding REPL in HTTP servers (e.g., accessible via curl).

Conclusion


The repl module is a powerful tool for interactive development and debugging in Node.js. With features like auto-completion, history, top-level await, and custom commands, REPL provides a flexible environment for experimenting with code and managing live applications.


Written & researched by Dr. Shahin Siami

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