What is JavaScript?

HTML is the structure, CSS is the look, and JavaScript is the behavior. It is a programming language that runs in the browser and can change the page while someone is using it.

Your first script

JavaScript goes inside a <script> element. console.log() prints a message to the console, a log that developers use to check what their code is doing. In these examples the console appears under the result.

<script>
console.log("Hello from JavaScript!");
console.log(2 + 3);
</script>

Changing the page

JavaScript can find an element and change it. Here, clicking the button runs a function, a named group of instructions.

<p id="message">Nobody has clicked yet.</p>
<button onclick="sayHi()">Click me</button>

<script>
let clicks = 0;

function sayHi() {
  clicks = clicks + 1;
  document.getElementById("message").textContent =
    "You clicked " + clicks + " times.";
  console.log("Click number", clicks);
}
</script>

What happened

• document.getElementById("message") finds the element whose id is message.
• .textContent = ... replaces the text inside it.
• onclick="sayHi()" tells the button to run the sayHi function when it is clicked.