Node.js REPL: From Basics to Advanced

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Node.js REPL (Read-Eval-Print Loop) is an interactive shell that allows you to run JavaScript code directly in a Node.js environment. This powerful tool is invaluable for debugging, prototyping, and learning JavaScript.

“Any fool can write code that a computer can understand. Good programmers write code that humans can understand.” — Martin Fowler

Starting the Node.js REPL

To launch the Node.js REPL, open your terminal or command prompt and type node followed by the Enter key. You should now see a > prompt, indicating that you're in the REPL.

$ node
>

Basic Usage

Type JavaScript code directly into the REPL, and it will execute the code and print the result.

> const x = 5;
undefined
> const y = 7;
undefined
> console.log(x + y);
12
undefined

Multiline Input

If you need to write a multiline expression, simply press Enter after each line. The REPL will automatically recognize when the expression is complete.

> function greet(name) {
...   return `Hello, ${name}!`;
... }
undefined
> greet('Alice');
'Hello, Alice!'

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Using the Underscore Variable

The result of the last executed expression is stored in the _ (underscore) variable. You can use this variable in subsequent expressions.

> const z = _ * 2;
24
> console.log(z);
24
undefined

Loading External Modules

You can also load external modules in the REPL using require().

> const fs = require('fs');
undefined

Saving and Loading REPL Sessions

To save your current REPL session to a file, use the .save command followed by the file path.

> .save my-session.js
Session saved to: my-session.js

To load a saved REPL session, use the .load command followed by the file path.

> .load my-session.js
Session loaded from: my-session.js

The Node.js REPL is a powerful tool for debugging, prototyping, and learning JavaScript. As you become more familiar with its features and capabilities, it will become an indispensable part of your Node.js development workflow.

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