Node.js REPL

The Node.js REPL is an interactive environment built directly into Node.js. It allows code to be typed and executed immediately, one line at a time, without needing to create a file. REPL stands for Read-Eval-Print Loop — and understanding each part of that name explains exactly what it does.

What Does REPL Stand For?

• Read: Node.js reads the input typed by the user.
• Eval: Node.js evaluates (runs) that input as JavaScript code.
• Print: Node.js prints the result of the evaluation to the screen.
• Loop: This process repeats — Node.js keeps waiting for the next input.

Think of the REPL as a conversation with Node.js. Type something in, and Node.js responds instantly. It is an excellent tool for testing small pieces of code, exploring built-in modules, and learning how JavaScript behaves in the Node.js environment.

How to Start the REPL

Open the terminal and simply type:

node

The terminal prompt changes to a greater-than sign (>), indicating that the REPL is active and ready for input:

Welcome to Node.js v20.11.0.
Type ".help" for more information.
>

Basic Usage Examples

Arithmetic Operations

> 5 + 3
8
> 10 * 4
40
> 100 / 5
20
> 7 % 3
1

Working with Variables

> let name = "Alice"
undefined
> name
'Alice'
> let age = 25
undefined
> age + 5
30

Note: When a variable is declared, the REPL prints undefined because the declaration itself does not produce a value. When the variable name is typed alone, its stored value is printed.

String Operations

> "Hello" + " " + "World"
'Hello World'
> "node".toUpperCase()
'NODE'
> "JavaScript".length
10

Using Built-in Objects

> Math.max(10, 20, 30)
30
> Math.sqrt(49)
7
> Date()
'Sat Mar 14 2026 10:00:00 GMT+0000'

Multi-line Code in REPL

When code spans multiple lines (like functions or loops), the REPL automatically detects that more input is needed and shows ... as a prompt:

> function greet(name) {
...   return "Hello, " + name + "!";
... }
undefined
> greet("Bob")
'Hello, Bob!'

The REPL waits until the closing brace is entered before evaluating the function definition.

REPL Special Commands

The REPL has several built-in commands that begin with a dot (.). These are called dot commands and they control the REPL itself rather than running JavaScript code.

Using the Underscore (_) for Last Result

In the REPL, the underscore character _ holds the result of the last evaluated expression. This is useful for chaining calculations:

> 10 + 5
15
> _ * 2
30
> _ - 6
24

Accessing Node.js Modules in the REPL

Built-in Node.js modules can be loaded directly in the REPL using require(). For example, to use the os module to get information about the operating system:

> const os = require('os')
undefined
> os.platform()
'linux'
> os.hostname()
'my-computer'

Exiting the REPL

There are three ways to exit the REPL:

• Type .exit and press Enter.
• Press Ctrl + C twice.
• Press Ctrl + D once.

When to Use the REPL

• Testing a small piece of code quickly without creating a file.
• Exploring how built-in methods or modules behave.
• Doing quick calculations or string manipulations.
• Learning and experimenting with JavaScript behavior in Node.js.

The REPL is not intended for writing full applications — it is a scratchpad for quick experimentation. For building real programs, JavaScript code is written in .js files and run using the node command.

Key Points

• REPL stands for Read-Eval-Print Loop — it reads input, runs it, prints the result, and repeats.
• Start the REPL by typing node in the terminal.
• Variables, functions, expressions, and modules can all be used directly in the REPL.
• Dot commands like .help, .exit, and .save control the REPL session.
• The _ variable stores the result of the last expression.
• The REPL is a powerful learning and testing tool but is not used for building complete applications.