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Introduction to JavaScript

Setting Up and Running Code

Introduction to JavaScript 15 min read

Setting Up and Running Code

Objectives

By the end of this chapter, you should be able to:

  • Run JavaScript in your browser’s console
  • Install Node.js and run a script from a file
  • Write and run your first JavaScript program

💡 Why this matters: Every lesson in this course asks you to run code. You need a place to actually do that first.

Quickest Option: The Browser Console

Every browser has a JavaScript console built in, and it’s the fastest way to try a line of code with zero setup.

  1. Open your browser.
  2. Open Developer Tools: right-click anywhere on a page and choose Inspect, or press F12 (Windows/Linux) or Cmd+Option+I (Mac).
  3. Click the Console tab.
  4. Type a line of JavaScript and press Enter:
javascript
2 + 2

You should see 4 printed back. The console is great for quick one-off checks, which is exactly what you’ll use it for in this lesson and the next.

Running a Script from a File: Node.js

The console works line by line, but real programs live in files. For that, install Node.js, which lets you run a .js file directly from your terminal.

  1. Download Node.js from nodejs.org (choose the LTS version).
  2. Confirm the install by running this in your terminal:
bash
node --version
  1. Create a file called first.js with one line:
javascript
console.log("Hello, JavaScript");
  1. Run it:
bash
node first.js
# Hello, JavaScript

console.log() prints a value. You’ll use it constantly to check what your code is actually doing.

Two Ways to Run Code

Like most languages, Node gives you both a REPL and a script runner.

The REPL (type node with nothing after it) is an interactive prompt, similar to the browser console: type a line, see the result immediately.

bash
node
> 2 + 2
4
> .exit

A script is a .js file holding your whole program, run with node filename.js. This is how the rest of this course expects you to work: write code in a file, then run the file.

Try It

  1. Open your browser console and run 10 * 4.
  2. Install Node.js and confirm it with node --version.
  3. Create first.js with console.log("Hello, JavaScript") and run it with node first.js.

Recap

  • The browser console is the fastest way to try a single line of JavaScript.
  • Node.js runs JavaScript files from your terminal with node filename.js.
  • console.log() prints a value so you can see what your code is doing.

Next lesson: variables, data types, and the operators you’ll use to work with them.