2 Running a Node.js application

This chapter introduces the fastest path to seeing JavaScript run outside the browser: writing a small script, executing it with Node.js from the terminal, and experimenting interactively in the REPL. You’re guided from a simple “print to the console” program to loading code into the REPL for quick testing, all to establish a smooth feedback loop for learning and debugging. Along the way, the chapter highlights how Node’s console output parallels browser debugging and encourages a disciplined setup with strict mode to catch common mistakes early.

You begin by creating a JavaScript file that logs a message, then run it from the command line with the node command (with or without the .js extension) from the file’s directory. If the script runs correctly, its message appears in the terminal, illustrating how console.log writes to standard output in Node. The chapter also recommends enabling strict mode (by placing 'use strict'; at the top of files) to prevent pitfalls like accidental globals or invalid assignments, reinforcing good habits as you start building applications.

Next, you move into the Node REPL for rapid, line-by-line experimentation. You can load existing code into a session with .load filename, use the imported variables immediately (for example, iterating over an array of messages and logging each one), and then save your interactive work back to a file with .save filename (note that it overwrites existing files). Practical tips include starting the REPL in the same directory as your files or using absolute paths, and exiting with .exit, Ctrl-C twice, or Ctrl-D twice. The chapter closes by encouraging practice with console output, including string interpolation, to build familiarity with Node’s debugging-friendly workflow.

Running a JavaScript file with Node.js

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