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This post is part of my Today I learned series in which I share all my web development learnings.

Today I came across a quick video which explains Node.js' REPL functionality (Read-Eval-Print loop).

While I use the built-in REPL from time to time (type node into your terminal to start it) to prototype code, I haven't used the REPL module before. It turns out that you can create custom REPLs with just a few lines of JavaScript:

// index.js
const repl = require('repl');

// define available methods and state
const state = {
  printSomething() {
    console.log("That's awesome!");

const myRepl = repl.start("stefan's repl > ");

Object.assign(myRepl.context, state);

If you're like me and like to prototype in a console, the repl package is handy. You could create an application entry script that provides all the initialized objects and functionality. Or you could even build a little shell for everyday tasks.

Screenshot of a terminal: start a new REPL by running "node index.js" and see the available command 'printSomething'

Let me know what REPL's you're building. I'd love to see more examples and real-world use cases. And now, let's "just REPL away"! 🎉

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Stefan standing in the park in front of a green background

About Stefan Judis

Frontend nerd with over ten years of experience, freelance dev, "Today I Learned" blogger, conference speaker, and Open Source maintainer.

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