JavaScript is a text-based programming language used in both client and server-side that allows web pages to be interactive. HTML and CSS are languages that give structure and style to web pages, JavaScript gives web pages interactive elements that engage a user.    
JavaScript enhance user experience of web pages by converting it from a static page into an interactive one. 

Bootstrap is the most popular, free and open-source framework for creating responsive layout in web pages, with much less efforts. It contains HTML, CSS and JS components for creating forms, buttons, navigation, dropdown, modals, layout and many other things.
Web designers and developers love to use Bootstrap in their projects. They use it to create a responsive web design that looks perfectly accurate on all screen sizes (smartphones, tablets, laptops and PCs).
If you are a web developer then you should look into the advantages that Bootstrap provides:
•    Saves you time – Quickly create features using predefined classes and design templates that are provided by bootstrap.
•    Responsive Design – With Bootstrap you don’t need to apply media queries in your CSS file. It does the adjustment of web page dynamically on all screen sizes.
•    Compatible with all browsers – You don’t have to worry about any browser as it is compatible with latest versions of all browsers – Google Chrome, Firefox, Opera, Safari and Edge.
•    Easy and Simple – It is very easy and simple to use in web designing. If you have basic knowledge of HTML and CSS then you should go ahead with it.
•    Consistency – It gives you the consistency between your projects and other developers.