How to Run JavaScript

In short: there are three everyday places to run JavaScript, and each fits a different job. The browser console runs whatever you type, instantly — for trying an idea. A <script> tag in an HTML page ships code to real visitors, and defer makes sure the page's elements exist before the script looks for them. Node.js runs a .js file with no browser at all, for servers and command-line tools. Wherever it runs, console.log(value) prints to that place's console — never onto the page itself.

Check a quick result in the console

Every browser's developer tools (usually F12, or right-click → Inspect) has a Console tab that runs whatever you type, right away. Nobody else needs the answer, so there is no reason to build a page for it:

In the real console you would type 214 / 60 directly and read the answer underneath it. A run block behaves the same way: it runs your code and shows what it prints.

Ship code to page visitors with a script tag

Code that a page's visitors must run every time they load it belongs in a file, loaded with a script tag:

defer tells the browser to finish reading the HTML before running the script. Without it, a script in <head> runs before the <body> exists, so document.querySelector("#play") returns null — the element it is looking for has not been read yet. Adding defer, or moving the script to the end of <body>, fixes it. This code needs a real page to run in, so it is shown here rather than run live.

Run a script with no browser at all

A job with no web page involved — renaming files, processing data, running a server — has no DOM to touch, so it runs with Node.js from a terminal instead:

Node executes the file directly and prints to the terminal. This is also how the servers behind most web apps are built: the same language, running with no browser underneath it.

Quick practice

🎯 Your turn

You want to (a) check what 214 % 60 gives, (b) make a playlist page's Shuffle button work for visitors, and (c) run a script on your laptop that renames music files. Explain where you would run the code in each case and why.

Write your explanation and submit it — there's no code to run here, just your reasoning.