1 · Get comfortable · 18 MIN

Run a JavaScript file outside the browser

Node.js runs JavaScript with server and operating-system APIs.

The browser provides window and document for working with a page. Node.js provides process and modules for work such as files and network servers; it does not create a browser DOM. A .mjs file uses JavaScript modules. Run it from a terminal and read its printed output there. process.argv contains the executable, script path and additional arguments; index 2 is the first value you type after the filename. Those arguments are strings until you convert them.

Check node --version, then save the .mjs file. Run node lesson.mjs Ada in the folder containing it. Compare argv[2] with the first extra argument. Read argv[3] for the optional topic.

Before you start

Install Node.js 22.12+ (or a supported newer LTS). Create a folder named codingneed-node. Open a terminal in that folder. Save ONE lesson example as lesson.mjs and run node lesson.mjs. No npm dependencies are required. HTTP lessons keep running until you press Ctrl+C; stop the previous server before starting another.

File for this example: lesson.mjs

New words, explained

runtime: The program that executes JavaScript and supplies APIs.
terminal: A text interface for starting programs.
argument: A value supplied when invoking a program or function.

Follow the example step by step

Check node --version, then save the .mjs file.
Run node lesson.mjs Ada in the folder containing it.
Compare argv[2] with the first extra argument.
Read argv[3] for the optional topic.

You are ready to move on when: The script runs without arguments. Two arguments appear in the greeting. Explain why document.querySelector is unavailable here.

Read the example

// Save this complete file as lesson.mjs.
const name = process.argv[2] ?? 'learner';
console.log('Hello, ' + name);
console.log('Arguments are ' + typeof name);
console.log('This program runs in Node.js, not in a browser page.');

Check the expected output

node lesson.mjs Ada prints Hello, Ada, then Arguments are string, then the runtime explanation. Without Ada the greeting uses learner.

Your challenge

Accept a second argument for a learning topic and include it in the greeting. Supply a default topic when omitted.

Solution cost: Discuss the operations in this small example; rendering and I/O costs depend on the host. time · Proportional to the example’s retained data. space

Common trap

Running code in a terminal is different from typing code into the operating-system shell.

Study the project implementation

// Save this complete file as lesson.mjs.
const name = process.argv[2] ?? 'learner';
console.log('Hello, ' + name);
console.log('Arguments are ' + typeof name);
console.log('This program runs in Node.js, not in a browser page.');

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