Node.js REPL: Read, Evaluate, Print, Loop

Ever needed to prototype or debug JavaScript without creating files? The Node.js REPL lets you:

• Instantly run snippets
• Explore APIs interactively
• Debug in real time

It operates in four steps:

1. Read your input
2. Evaluate the code
3. Print the result
4. Loop back for more

Prerequisites

• Node.js installed

Getting Started

Start & Exit

$ node

You'll see:

Welcome to Node.js v22.14.0.
Type ".help" for more information.
>

Exit by:

• .exit
• Ctrl+D (once)
• Ctrl+C (twice)

Try Simple Expressions

> 5 + 6
11
> "Hello" + " World"
'Hello World'
> 1 === "1"
false

Variables & Functions

> let x = 10
undefined
> x * 2
20

> function sum(a, b) {
...   return a + b;
... }
undefined
> sum(3, 4)
7

Special “_” Variable

The underscore _ stores the last result:

> 2 + 3
5
> _
5

Multi‑line Mode

When you enter an incomplete statement, REPL switches to multi‑line:

> if (true) {
...   console.log('Hi');
... }
Hi

REPL Shortcuts & Commands

Shortcuts:

• Up/Down arrows: Browse history
• Tab: Auto‑complete
• Ctrl+C: Cancel current input
• Ctrl+D: Exit

Built‑in commands (start with .):

Command	Description
.help	Show help
.break/.clear	Abort multi‑line input
.editor	Enter editor mode
.save	Save session to <file>
.load	Load and run <file>
.exit	Exit REPL

Save & Load Example

> let a = 5, b = 10
> .save session.js
Session saved to: session.js

In a new session:

$ node
> .load session.js

Summary

• Launch with node.
• Use REPL for quick feedback.
• Leverage .help, .save, .load.
• Navigate with arrows and Tab.
• Harness _ for the last result.

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Global Objects in Node.js

What Is the Global Object?

In JavaScript, the global object holds values available everywhere.
- Browsers: window
- Node.js: global or standardized globalThis

Browser vs. Node.js

Browser:

window === this          // true
window.setTimeout === setTimeout // true

Node.js:

global.setTimeout === setTimeout // true
this                              // {} (module scope)
globalThis === global            // true

Common Globals in Node.js

// Timers
setTimeout(fn, ms)
setInterval(fn, ms)
clearTimeout(id)

// Console
console.log()
console.error()
console.time(label); console.timeEnd(label)

// Buffer
const buf = Buffer.from('hello')

// Process
process.env
process.argv
process.cwd()
process.exit(0)

// Module info
__dirname
__filename

Module Scope vs. Global Scope

Each file is its own module. Top‑level let, const, or var stay local:

// moduleA.js
const local = 'module-only';
global.globalVar = 'truly global';

console.log(local);       // ReferenceError
console.log(globalVar);   // 'truly global'

To share, attach to global or export via module.exports.

Using globalThis

globalThis works uniformly across environments:

globalThis.myValue = 42;
console.log(globalThis.myValue); // 42

Best Practices

• Avoid polluting the global namespace.
• Prefer module exports/imports over globals.
• Use process.env for configuration.
• Reserve globals for universal utilities (e.g., polyfills).