why there are different kinds of js libraries like node js, angular js, react js etc.

To reduce size, complexity,load time of a website or webpage used for different porpose

They are not libraries. Node.js is a runtime environment. The rest are frameworks. Each one of them has different implementations and purposes. You can read more about their differences here https://en.m.wikipedia.org/wiki/Comparison_of_JavaScript_frameworks Edit: if you want to know what a framework is in the first place, read this: https://skillcrush.com/blog/what-is-a-javascript-framework/#whatis