Table of Contents

• REPL
  • Design and Features
    • Commands and Special Keys
    • Default Evaluation
      • JavaScript Expressions
      • Global and Local Scope
      • Accessing Core Node.js Modules
      • Assignment of the _ (underscore) variable
    • Custom Evaluation Functions
      • Recoverable Errors
    • Customizing REPL Output
  • Class: REPLServer
    • Event: 'exit'
    • Event: 'reset'
    • replServer.defineCommand(keyword, cmd)
    • replServer.displayPrompt([preserveCursor])
  • repl.start([options])
  • The Node.js REPL
    • Environment Variable Options
    • Persistent History
      • NODE_REPL_HISTORY_FILE
    • Using the Node.js REPL with advanced line-editors
    • Starting multiple REPL instances against a single running instance

REPL#

The repl module provides a Read-Eval-Print-Loop (REPL) implementation that is available both as a standalone program or includible in other applications. It can be accessed using:

const repl = require('repl');

Design and Features#

The repl module exports the repl.REPLServer class. While running, instances of repl.REPLServer will accept individual lines of user input, evaluate those according to a user-defined evaluation function, then output the result. Input and output may be from stdin and stdout, respectively, or may be connected to any Node.js stream.

Instances of repl.REPLServer support automatic completion of inputs, simplistic Emacs-style line editing, multi-line inputs, ANSI-styled output, saving and restoring current REPL session state, error recovery, and customizable evaluation functions.

Commands and Special Keys#

The following special commands are supported by all REPL instances:

• .break - When in the process of inputting a multi-line expression, entering the .break command (or pressing the <ctrl>-C key combination) will abort further input or processing of that expression.
• .clear - Resets the REPL context to an empty object and clears any multi-line expression currently being input.
• .exit - Close the I/O stream, causing the REPL to exit.
• .help - Show this list of special commands.
• .save - Save the current REPL session to a file: > .save ./file/to/save.js
• .load - Load a file into the current REPL session. > .load ./file/to/load.js
• .editor - Enter editor mode (<ctrl>-D to finish, <ctrl>-C to cancel)

> .editor
// Entering editor mode (^D to finish, ^C to cancel)
function welcome(name) {
  return `Hello ${name}!`;
}

welcome('Node.js User');

// ^D
'Hello Node.js User!'
>

The following key combinations in the REPL have these special effects:

• <ctrl>-C - When pressed once, has the same effect as the .break command. When pressed twice on a blank line, has the same effect as the .exit command.
• <ctrl>-D - Has the same effect as the .exit command.
• <tab>