Each of these languages is used for a different purpose and together they are used to build a website with all desired functionalities.
- Access an element on the website and change its characteristics based on your criteria(DOM) for e.g. Using in online forms, "asking user to fill in all required fields before submitting form to the server".
- Talk to server without refreshing the web page (AJAX)
- Fix browser issues such as IE compatibility problems
- Animate elements on your website using JQuery (A library of JS)