Your First Run: The Console, Node, and the Playground

Zara does not trust anything she has not tested herself. So before she writes a real program, she runs one line, console.log("Hello");, in three different places. A browser, a file on her laptop, and the Playground.

All three print Hello. That is the point of this lesson. JavaScript is one language, and after today you will know three places to run it, so you never have to wonder where code goes.

Pick whichever place is easiest for you now. The Playground needs nothing installed, and every lesson's Run button uses it.

Place one: the browser console

Every desktop browser hides a console inside its developer tools: a text box where you type JavaScript and see the result at once. It is the fastest place to try one line.

Any of them also opens with F12; then click the tab called Console. Click in the box at the bottom, type console.log("Hello"); and press Enter. You will see two lines come back:

Hello
undefined

The first line is your output. The grey undefined is the console telling you that the line itself gave back no value. It is not an error; every console does it, and you can ignore it.

One more surprise. The first time you paste code into Chrome's or Firefox's console, it may refuse and ask you to type allow pasting. This protects people from scammers who say "paste this into your console". Type it once, and pasting works.

So the browser console runs one line the moment you press Enter.

Place two: Node.js, from a file

The console is perfect for one line and awkward for twenty. For a real program you save it in a file and run the file with Node.

1. Install Node.js from nodejs.org. Pick the version marked LTS (long-term support). Any version from 22 up runs everything in this track.
2. Open a terminal and type node --version. It should print a version starting with v22 or higher.
3. In any text editor, save the line console.log("Hello"); in a file called hello.js.
4. In the terminal, go to the folder with that file and type node hello.js.

The terminal prints Hello, with no undefined after it. Only the console adds that extra line; a program run from a file prints exactly what it logs.

Typing node alone opens Node's own console, called the REPL (read, evaluate, print, loop). It works like the browser console. Leave it with .exit.

So Node runs a whole file with one command, node and the file's name.

Place three: the Playground

The Playground runs the same Node.js 22 on a Progsity machine, so you install nothing. Every Run link in these lessons opens it with the example already typed in. To start from a blank page, choose the language JavaScript (Node.js), version Node.js 22.

Four parts of the screen matter:

• The editor, where your code goes.
• Run, which sends the code to the machine and runs it.
• Standard Input (stdin), a box for text your program can read. You type it before you press Run.
• The output, with a badge saying how the run ended: Success, Compile error or Runtime error, as lesson 04 explained.

The input box matters from Module 1 on. Every judged problem gives your program its input through stdin, and Example 4 below takes a first look.

So the Playground is Node.js 22 with an input box and an output pane, one click away.

What console.log prints

console.log prints whatever you give it, then moves to a new line. You give it things between its round brackets, separated by commas. These things are called arguments.

console.log("Total:", 250 + 120);

Three rules cover nearly everything. Text in quotes is printed without its quotes. A number or a sum is printed as its value. Several arguments are printed on one line with one space between each.

console.log("Hello");

Hello

console.log("Zara", 3);
console.log("Total:", 250 + 120);
console.log();
console.log("a", "b", "c");

Zara 3
Total: 370

a b c

Other ways to show output

console.log is not the only way a program can show something, but it is the only one that works in all three places.

So console.log is the one output tool this track relies on, because it works everywhere.

Semicolons and comments

A statement is one complete instruction, such as one console.log(...) call. This track ends every statement with a semicolon, ;. JavaScript can often add a missing one by itself, but not always, and Module 1 shows the one case where that bites. Writing them yourself removes the question, so this is said once and then simply done.

A comment is text the engine skips completely. It is a note for the human who reads the code next, often you a month later. There are two forms: // comments out the rest of a line, and /* ... */ comments out everything between the two marks, across several lines if needed.

// Zara's first program: two lines of output.
console.log("Hello"); // a comment can follow the code
/* A comment can also
   cover several lines. */
console.log("Goodbye");

Hello
Goodbye

const input = require("fs").readFileSync(0, "utf8");
console.log("You typed: " + input.trim());

You typed: Zara

Console.log("Hi");

console.log(Hello);

console.log(“Hi”);

console.log(1, 2);
console.log("1" + 2);

Name: Zara
Track: JavaScript
Lesson: 05

Apples: 12

Module 1 starts writing real programs: numbers, text, names for values, and reading input the way every judged problem does.