The REPL and Running Scripts

What is the REPL?

The REPL is an interactive prompt where you type JavaScript and see the result straight away. The name describes what it does, over and over:

• Read the line you type.
• Evaluate (run) it.
• Print the result.
• Loop back and wait for the next line.

It is like a calculator for JavaScript. You do not write a file or save anything. You just ask a question and get an answer. It is perfect for checking how a function behaves before you use it in a real script.

Starting and leaving the REPL

Type node on its own and press Enter:

node

Welcome to Node.js v24.12.0.
Type ".help" for more information.
>

The > is the prompt. It means Node is waiting for you.

To leave, use any one of these:

• Type .exit and press Enter.
• Press Ctrl + D.
• Press Ctrl + C twice. The first press clears the current line and reminds you how to exit.

Trying things out

Type an expression and press Enter. The REPL prints its value:

> 2 + 3
5
> 'asha'.toUpperCase()
'ASHA'
> [1, 2, 3].map(n => n * 2)
[ 2, 4, 6 ]
> Math.max(4, 9, 2)
9

You never needed console.log. The REPL prints every result automatically.

Why does it print undefined?

Some lines do not produce a value. Declaring a variable is one of them, so the REPL prints undefined:

> const name = 'Ravi'
undefined
> `Hello, ${name}`
'Hello, Ravi'

console.log also returns nothing. You see the logged text, then undefined as the return value:

> console.log('hi')
hi
undefined

This is normal, not an error.

Multi-line code

If a line is unfinished, such as a function with an open {, the REPL waits for more. The prompt changes to | until the code is complete:

> function add(a, b) {
| return a + b
| }
undefined
> add(2, 3)
5

Handy REPL features

The last result: _

The underscore holds the result of the previous line:

> 19.99 * 3
59.97
> _ + 4.99
64.96

Likewise, _error holds the last error that was thrown.

History and completion

• Press the Up and Down arrows to scroll through lines you typed before, even from earlier sessions.
• Press Tab to complete names. Type Math. and press Tab twice to see everything on Math.

Built-in modules are already loaded

In the REPL, Node’s built-in modules are available by name without importing them:

> os.platform()
'linux'
> path.join('notes', 'todo.txt')
'notes/todo.txt'

This only works in the REPL. In a script file you must import them.

Top-level await

You can use await directly at the prompt:

> await new Promise(resolve => setTimeout(() => resolve('done'), 500))
'done'

Dot commands

Lines that start with a dot are commands for the REPL itself, not JavaScript. Type .help to see them all:

.save is a nice way to turn an experiment into a starting script.

Running scripts

The REPL is for quick experiments. Real programs live in files, and you run them by passing the file name to node:

node app.js

Anything after the file name is passed to your program in process.argv, as you saw in Your First Node.js Program:

node app.js Asha --verbose

Running a one-liner with -e

-e (short for --eval) runs a snippet of code without creating a file:

node -e "console.log('Hi from -e')"

Hi from -e

Printing a value with -p

-p (short for --print) runs an expression and prints the result, like the REPL does:

node -p "2 + 3"
node -p "process.versions.v8"

5
13.6.233.17-node.37

Tip: Wrap the code in double quotes and use single quotes inside it. That works in Bash, Zsh, PowerShell and the Windows Command Prompt.

Checking syntax with --check

--check (or -c) reads a file and reports syntax errors without running it. Useful before running a script that changes files:

node --check app.js

If the file is fine, it prints nothing. If not, it shows where the problem is:

app.js:1
console.log("x"
            ^^^

SyntaxError: missing ) after argument list

Re-running on every save with --watch

--watch runs your script and restarts it every time you save the file:

node --watch app.js

Press Ctrl + C to stop watching. Watch Mode with node –watch covers it in depth.

Where options go

node --version (or -v) prints the version and node --help (or -h) lists every option. Options for Node itself go before the file name. Arguments for your program go after it:

node --watch app.js --verbose
#    ^ for Node   ^ for app.js

Common mistakes

Typing a terminal command inside the REPL

> node app.js
Uncaught SyntaxError: Unexpected identifier 'app'

The REPL only understands JavaScript. Leave with .exit first, then run node app.js from your normal terminal prompt.

Declaring the same const twice

> const total = 10
undefined
> const total = 20
Uncaught SyntaxError: Identifier 'total' has already been declared

The REPL remembers every variable for the whole session, just like one long script. Pick a new name, use let and reassign it (total = 20), or restart the REPL.

Quick recap

• The REPL (Read, Evaluate, Print, Loop) starts when you run node with no file.
• It prints each result automatically. undefined just means “no value”.
• _ is the last result, Up arrow is history, Tab completes names.
• Dot commands like .help, .editor, .save and .exit control the REPL.
• Leave with .exit, Ctrl + D or Ctrl + C twice.
• Run files with node file.js; use -e to run code, -p to print a value, --check for syntax and --watch to restart on save.
• Node options go before the file name, your program’s arguments go after it.