Running a Node Program

The Two Modes of Execution

You wouldn’t try to drive a car by turning the key and then pushing it down the street, right? (Don’t answer that). In Node.js, we have two distinct ways to get our code running. Understanding the difference is the first step in not looking like a total rookie.

1. The REPL (Read-Eval-Print Loop)

This is the “Sandbox” mode. It’s great for quick tests, doing math, or checking if a function does what you think it does. You type node in your terminal, hit enter, and you’re in an interactive shell.

[!NOTE] Think of the REPL as a scratchpad. It doesn’t save your work. Once you exit, it’s gone forever, like a Snapchat message or my patience for unformatted code.

2. Script Execution (The “Real” Way)

This is how we build actual applications. We write our code in a file (like app.js), save it, and then tell Node’s V8 engine to “ingest” that file and execute every line inside it one by one.

This is the Ignition Sequence:

Fig 1. The Node.js Process Lifecycle (Artist’s Dramatization)

The node [filename] Command

To run a script, you need to use the node command followed by the relative path to your file.

If your file is named app.js, you simply run:

Terminal

node app.js

When you hit enter, Node:

1. Reads the file from your hard drive.
2. Parses the JavaScript.
3. Executes it.
4. Exits immediately when there is no more work to do (unless an Event Loop keeps it alive—more on that later).

The “Hello Node” Moment

If you’ve done everything right, your terminal should look something like this. Bask in the glory of your first server-side output.

Common Pitfalls (Read This or Weep)

• Wrong Directory: If you see Error: Cannot find module..., you are in the wrong folder. Use ls (lines) or dir (windows) to check if your file actually exists where you are standing.
• Missing Extension: You can technically run node app, but please don’t. Be explicit. Use node app.js.
• The Hanging Process: If your terminal doesn’t give you the prompt back, your code is still running (probably an infinite loop or an open server). Ctrl + C is your escape hatch.

⏭ nodemon: The Watcher in the Code

Save your fingers from typing node app.js 500 times a day