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Explain the concept of REPL in Node.js

Q: Explain the concept of REPL in Node.js

**TL;DR.** **REPL** = **Read-Eval-Print Loop**: an interactive shell that **reads** an expression, **evaluates** it, **prints** the result, and **loops**. Start it by running node with no file. It's great for quickly trying APIs, inspecting values, and prototyping; you can also build **custom REPLs** with the repl module. Read Eval P

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A fundamental question about the Node.js interactive environment often asked in early screening rounds.

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Interactive session + a custom REPL

// $ node
// > const x = 21
// > x * 2
// 42
// > _            // last result
// 42

// Embed a custom REPL with context pre-loaded:
import repl from 'node:repl';
const r = repl.start('app> ');
r.context.db = db;           // now you can use 'db' in the prompt

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