JavaScript · Lesson 1 of 8

JavaScript introduction

What JavaScript does, where to write it and console.log.

JavaScript (JS) runs in the browser and makes pages interactive: reacting to clicks, validating forms, loading data. It also runs on servers through Node.js.

Add JS with a <script> tag — usually a file: <script src="app.js" defer></script>. defer runs it after the HTML has loaded.

console.log() prints values to the console — your main debugging tool. In the Try-it editor, console output appears under the preview.

Try it yourself

Code editor

Edit the code and press Run (or Ctrl/⌘ + Enter). Code runs only in your browser, in a sandbox.

Key points

JS adds behaviour to HTML.
Use defer when loading scripts in <head>.
console.log is for debugging.

Quick check

What does console.log(2 + 3) print?

23 5 2 + 3 undefined

Practice task

Print your name and today's date to the console, then change the page heading to 'I am learning JavaScript'.

Interview corner

What is the difference between defer and async on a script?

Both download in parallel. defer runs after HTML parsing, in order; async runs as soon as it downloads, in any order.

Is JavaScript the same as Java?

No. They are different languages with different designs; the similar name was a marketing choice.