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Your First Program

Scope: running a script with node, reading a file, and understanding the two module systems you'll meet. All commands below were run and verified with Node v24.

Run a script

Create hello.mjs:

console.log("Hello, world!");

node hello.mjs

Check: prints Hello, world!

console.log is how you print. The .mjs extension means "this file uses ES modules" — more on that below.

Do something real: read a file

Create readit.mjs:

import { readFile } from "node:fs/promises";

const text = await readFile("hello.mjs", "utf8");
console.log("File has", text.length, "characters");

node readit.mjs

Check: prints something like File has 32 characters.

Two things to notice:

  • node:fs/promises — the node: prefix marks a built-in module (files, in this case). The /promises version returns promises instead of using callbacks — the modern way.
  • Top-level await — you can use await directly in a module file, no wrapper function needed. (This only works in modules, which is one reason we use .mjs.)

The two module systems (read this once)

JavaScript has two ways to share code between files, and you will meet both:

ES modules (modern) CommonJS (older)
File extension .mjs .cjs (or .js)
Import import { x } from "..." const { x } = require("...")
Export export function x() module.exports = ...

Use ES modules (.mjs, import) for everything new. You'll still read CommonJS in older libraries and tutorials — now you know what you're looking at.

The plain .js extension is ambiguous: Node treats it as CommonJS unless the nearest package.json says "type": "module". Using .mjs/.cjs explicitly sidesteps the whole question. (See Modules and npm.)

Arguments and exiting

// args.mjs
console.log("You passed:", process.argv.slice(2));

node args.mjs one two

Check: prints You passed: [ 'one', 'two' ]

  • process.argv holds command-line arguments; the first two entries are the node binary and the script path, so real arguments start at index 2.
  • process.exit(1) ends the program with a failure code (0 = success, anything else = something went wrong).

The REPL

Type node with no arguments for an interactive prompt — handy for trying snippets:

$ node
> 2 + 2
4
> .exit

Press Ctrl+C twice or type .exit to leave.

Check

  • [ ] You can run a .mjs script and explain the extension
  • [ ] You can read a file with node:fs/promises and top-level await
  • [ ] You can recognize CommonJS (require) vs ES modules (import) in the wild

Next: Variables and Types.