- Detect if a form is filled out properly and let users know which field(s) need to be corrected
- Show/Hide different elements on the page
- Load information from an external source as needed
- Animate page elements
- Make calculations/conversions
JavasScript is also very popular for mobile app development through technologies like Angular.js. Using such a technology can save you quite a bit of time as opposed to building separate Java versions for IOS and Android etc.
Why should somebody use this technology over alternatives?