Running Your First Code

Where to type and run JavaScript, how programs execute top to bottom, and how to write comments.

Good news: you already have everything you need to run JavaScript. Every web browser ships with it.

Try this right now: press F12 (or right-click any page → Inspect), click the Console tab, type 2 + 2, and hit Enter. The browser answers 4. You just ran JavaScript.

Before we go further, one core idea: a program is simply a list of instructions. JavaScript reads them top to bottom, one line at a time, doing exactly what each line says — no more, no less. Every program you'll ever write builds on that simple rule.

Three common places to write and run JavaScript:

The browser console — great for quick one-line experiments (like the 2 + 2 you just tried)
Online playgrounds — sites like CodePen or StackBlitz give you an editor and instant results. The interactive exercises here on LingoCode work the same way
Files on your computer — real projects live in files ending in .js, run with Node.js or loaded by a web page

Predict the output

JavaScript runs top to bottom. What does this print?

console.log("one");
console.log("two");
console.log("three");

Aone two three
Bthree two one
Cone two three
DThe order is random

Sometimes you want to leave a note in your code — for your future self, or for teammates. That's what comments are for: lines the computer completely ignores.

// This is a comment — JavaScript skips this line entirely
console.log("This line runs"); // comments can also sit at the end of a line

/*
  This is a multi-line comment.
  Everything between the markers is ignored,
  no matter how many lines it spans.
*/

// — everything after it on that line is ignored
/ ... / — everything between the markers is ignored

Programmers also use // to temporarily switch a line off while testing — called commenting out a line.

Multiple choice

What does JavaScript do with a line that starts with //?

AIt ignores the line completely
BIt prints the line to the console
CIt runs the line twice
DIt throws an error

One more thing you'll meet on day one: error messages. Everyone — beginner and expert — sees them constantly. They're not scoldings; they're the computer telling you exactly what confused it.

console.lg("hello");
// TypeError: console.lg is not a function

The message says it plainly: there's no lg on console (we misspelled log).

Two habits that will save you hours:

Read the message — it usually names the exact problem and the line number
Check capitalisation — JavaScript is case-sensitive: console, Console, and CONSOLE are three different names, and only the first one exists

Fill in the blank

Add the marker that turns this line into a comment:

___ TODO: add a welcome message here
console.log("App started");

Predict the output

One line is commented out. What prints?

console.log("start");
// console.log("middle");
console.log("end");

Astart middle end
Bstart end
Cmiddle
DError

Recap — what you learned:

You can run JavaScript right now in the browser console (F12 → Console), in online playgrounds, or from .js files with Node.js
A program is a list of instructions, executed top to bottom, one line at a time
Comments (// and / /) are notes the computer ignores
Error messages are helpful, not scary — read them, and watch your capitalisation

Next: teaching your programs to remember things with variables.

// This is a comment — JavaScript skips this line entirely console.log("This line runs"); // comments can also sit at the end of a line /* This is a multi-line comment. Everything between the markers is ignored, no matter how many lines it spans. */