You can put Javascript right into html tags as with the "onclick" function with the html below
This is the paragraph with the ID attribute "demo", click the button below to see how this changes.
You can also put javascript in the document between "script" tags. Open the console to see the result of the javascript displayed in the script tags below
javascript can change the look of the page and elements on it. To see the script that runs this, look at the definition of myFunction() in lines 65-68, below.
Or you can call it in from another file. See the call for this function in the file referenced on line 25. It calls the file demo.js, located in the same folder as this file in the file tree.