Exploring with the REPL

Exploring with the REPL

So far, you have written code into a file and run it with the node command. That works well, but sometimes you just want to ask JavaScript a quick question: "What does this return?" or "Did I spell that method right?" For that, Node.js has the REPL.

REPL stands for Read, Evaluate, Print, Loop. It is an interactive conversation with JavaScript:

1. Read — Node reads what you typed.
2. Evaluate — Node runs it.
3. Print — Node shows you the result.
4. Loop — Node waits for your next line, then repeats.

Every modern language has a tool like this. Python calls its version the interpreter shell. The REPL is sometimes called a "playground" because it is perfect for trying things out with no files, no setup, and no risk to your project.

Starting and Leaving the REPL

Open your terminal and type:

node

You will see a prompt that looks like this:

Welcome to Node.js v20.x.x.
Type '.help' for more information.
>

That > is Node waiting for you. Type any JavaScript expression and press Enter:

> 2 + 2
4
> 'hello'.toUpperCase()
'HELLO'
> 5 > 3
true

Notice you do not need console.log. The REPL prints the result of every expression automatically. When you are done, press Ctrl+C twice (or type .exit and press Enter) to leave.

Things Worth Trying

The REPL remembers what you defined, so you can build up small experiments line by line:

> const name = 'Ada'
undefined
> `Hello, ${name}!`
'Hello, Ada!'
> [1, 2, 3].map(n => n * 10)
[ 10, 20, 30 ]

Two small notes on what you see:

• Assignments like const name = 'Ada' print undefined because the statement itself has no value.
• The REPL even prints after you press Enter on an incomplete line, letting you continue typing. It understands when your code is not finished, for example when you open a bracket and have not closed it yet.

Handy Shortcuts

The REPL has a few dot commands — special instructions that start with a period:

Two keyboard shortcuts are also worth knowing:

• Press the up arrow to bring back previous lines, like any terminal history.
• Press Tab to auto-complete. Type Math., press Tab, and the REPL lists everything the Math object offers — a fast way to discover functions.

When to Use the REPL

The REPL is a tool for questions, not for real programs. Use it when you want to:

• Check what an expression returns before putting it in your code.
• Explore how a built-in object, like Math or JSON, behaves.
• Do quick math or text conversions without opening an editor.

Anything longer than a few lines belongs in a file, where you can save it, edit it, and run it again. A good habit: experiment in the REPL, then copy what worked into your file.