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Node.js REPL

Imagine you're learning a new language and you want to practice speaking immediately without writing a full essay. That's exactly what REPL does for Node.js – it's an interactive playground where you can type JavaScript and see results instantly!

What is REPL?

REPL stands for Read-Eval-Print Loop. It's an interactive programming environment that:
  1. Reads your input (JavaScript code).
  2. Evaluates (executes) it.
  3. Prints the result.
  4. Loops back to wait for more input.

Think of REPL as a conversation with Node.js. You say something (write code), and Node.js responds immediately (shows output).

Starting REPL

Open your terminal and simply type:
node

You'll see a `>` prompt. Now you're in REPL mode!

Basic REPL Commands

CommandWhat It Does
`.help`Shows all available REPL commands.
`.break`Exit from multi-line input.
`.clear`Clear the REPL context (reset variables).
`.save filename.js`Save your REPL session to a file.
`.load filename.js`Load a file into the REPL session.
`.exit`Exit REPL (or press Ctrl+C twice).

Using REPL for JavaScript Practice

Let's try some examples:
> 5 + 3
8
> const name = 'Node.js';
undefined
> console.log(`Hello, ${name}!`);
Hello, Node.js!
undefined
> const numbers = [1, 2, 3, 4];
undefined
> numbers.map(n => n * 2);
[ 2, 4, 6, 8 ]

Notice that when you assign a variable, it returns `undefined`. When you evaluate an expression, it returns the result.

Using Modules in REPL

You can also require and use modules in REPL:
> const fs = require('fs');
undefined
> fs.readdirSync('.');
[ 'file1.txt', 'file2.js', 'folder' ]
> const path = require('path');
undefined
> path.extname('image.jpg');
'.jpg'

Multi-line Code in REPL

You can write multi-line code by using curly braces `{}`. REPL will show `...` indicating it's waiting for more input:
> function greet(name) {
... console.log(`Hello, ${name}!`);
... }
undefined
> greet('Node');
Hello, Node!

Saving and Loading REPL Sessions

This is a powerful feature. You can save your entire REPL session to a file:
> const x = 100;
> const y = 200;
> function add(a, b) { return a + b; }
> .save mysession.js
Session saved to: mysession.js

Later, you can load it back:
> .load mysession.js

Tab Completion

REPL supports tab completion. Type part of a variable or function name and press `Tab` to see suggestions:
> cons  <!-- Press Tab -->
console

When to Use REPL

  • Learning JavaScript: Test syntax and features instantly.
  • Debugging: Check values and test small code snippets.
  • Exploring APIs: Try out built-in modules without creating files.
  • Quick calculations: Use it as a calculator.

Two Minute Drill

  • REPL = Read-Eval-Print Loop – an interactive JavaScript environment.
  • Start REPL by typing `node` in terminal.
  • Use `.help` to see available commands.
  • Save sessions with `.save` and load with `.load`.
  • Perfect for testing, learning, and debugging.
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