Node.js REPL

What is REPL in Node.js?

Typing node with no arguments opens a Read-Eval-Print Loop: an interactive shell that reads an expression, evaluates it, prints the result and loops. It is the fastest way to test a snippet, explore an unfamiliar module or check what a function actually returns, with no file to create and delete.

Why REPL matters

Most "does this method return an array or an object?" questions take five seconds in the REPL and five minutes if you write a script for them. It is also how you inspect a running server’s modules without touching the codebase.

REPL example

$ node
Welcome to Node.js v22.
> const fs = require('node:fs')
undefined
> fs.readdirSync('.')
[ 'package.json', 'src', 'node_modules' ]
> _.length            // _ holds the previous result
3
> await fetch('https://api.github.com').then(r => r.status)
200
> .exit

How this works

Each line is evaluated immediately and its value printed — which is why const declarations print undefined. The special variable _ holds the last result, so you can chain exploratory steps without retyping.

REPL dot-commands

.help          list every command
.editor        multi-line editing mode (Ctrl+D to run)
.save out.js   write this session's history to a file
.load in.js    execute a file inside the current session
.break         abandon a half-typed multi-line expression
.clear         reset the REPL context
.exit          quit (or press Ctrl+D)

Key points to remember

• _ holds the value of the last evaluated expression.
• Tab completion works on object properties, so fs.<Tab> lists the whole API.
• Top-level await works, which makes testing promise-based code straightforward.
• .editor mode lets you paste or write multi-line functions without the REPL executing each line.
• History persists between sessions in ~/.node_repl_history.

Ways to run Node code, and when to use each

Common mistakes with REPL

• Pasting a multi-line function without .editor, so the REPL evaluates each line separately and errors.
• Expecting variables from the REPL to be visible to a separate node process — each session is isolated.