When you start programming/coding in computers, you will come across the need to write some code that needs to be used again and again.
In general a simple way is to write this code as functions and reuse them in your program.
In the field of computing programmers and developers observed that a lot of commonly required code is re-written again and again by people all over in the field of computing. So some decided to pack these pieces of code in pack of single/multiple files organized and categorized for different presuppose.
Programming
JavaScript is one the most commonly used scripting languages which primarily runs on you Web Browser. The primary use of JavaScript is to manipulate a HTML Page (along with CSS) based on users requirements (of course some has to program that!!).