JavaScript in the Browser

JavaScript is the one programming language that runs natively in every web browser, it's what makes pages interactive: reacting to clicks, fetching data, and updating content without a full page reload.

Your first JavaScript

const name = 'Kodstigen';
console.log('Hello, ' + name + '!');

const year = new Date().getFullYear();
console.log('It is', year);

Declaring variables

• const declares a variable that can't be reassigned after it's set, use it by default.
• let declares a variable you can reassign later, use it when a value needs to change (like a counter).
• var is the old way of declaring variables, from before let/const existed, you'll still see it in older code, but modern JavaScript avoids it.

let xp = 0;
xp = xp + 10; // fine, let can be reassigned

const name = 'Ada';
// name = 'Grace'; ❌ this would throw an error, const can't be reassigned

The DOM

In a real page, JavaScript reads and modifies the DOM (Document Object Model), the live tree of elements your code can change:

const button = document.querySelector('#greet');
button.addEventListener('click', () => {
  document.querySelector('#out').textContent = 'Hello!';
});

DOM code needs a real web page, so that block is read-only here, every other example in this course is runnable. Try the first one!