JavaScript is used to control the functionality of web pages - without JavaScript, a page would not have interactive elements and would always be static. JavaScript is added within the page source code and can be either loaded within `<script>` tags or can be included remotely with the src attribute: `<script src="/location/of/javascript_file.js"></script>` The following JavaScript code finds a HTML element on the page with the id of "demo" and changes the element's contents to "Hack the Planet" : `document.getElementById("demo").innerHTML = "Hack the Planet";` HTML elements can also have events, such as "onclick" or "onhover" that execute JavaScript when the event occurs. The following code changes the text of the element with the demo ID to Button Clicked: `<button onclick='document.getElementById("demo").innerHTML = "Button Clicked";'>Click Me!</button>` - onclick events can also be defined inside the JavaScript script tags, and not on elements directly