What problems does javascript frameworks like React/Angular solve?

Others have already mentioned the advantages and what these frameworks resolve.

I wanted to add what problems they can create:

1. I have seen many people that use a framework see it as a hammer and every website as a nail. In other words they either don't or can't figure out that in many cases a framework is not needed. I have seen web pages that are nearly 90% static data still download an entire framework just to provide drop menus or something else trivial.
2. Frameworks force you into a lifecycle and development style that, in some ways, make some things easier and other things much, much more complicated. AngularJS had a nasty digest cycle that often led to complications. Redux forces you to write much more code than needed just to handle state that on many pages is not needed. Vue, React, Angular all force you to either use their data creation functions, their way of rendering or their way of handling inter element communication. And none of these are faster than vanillaJS. Even virtual DOM has its issues.
3. The size of a framework is often much more than needed. I have several small libraries that do the majority of the work that a framework does but my code is in the sub 5K range if I load it all.
4. Some people that learn a framework first can not figure out how to write in raw JavaScript. They also tend to learn sloppy ways to write code. No, not everyone, but a large number people that did not start with vanilla JS write code that tends to be much larger and sloppier than needed.
5. It is best to avoid JavaScript except where really needed. Often I can use raw HTML and CSS to do everything I need to do. I find that it is rare that I need to use much JavaScript even in projects that exceed several thousand files. It is faster to allow the browser to process HTML and CSS, which all processes at compiled speeds and only use JavaScript for things you can not do in HTML and CSS.

These are my opinions. I have worked as a developer for almost 40 years and as a Senior Web Dev for since the days of IE3 and I stand by these opinions.

We all need to better learn raw JS to understand what a framework is doing and truly know if a framework is needed.

So go ahead and down vote me. But think about this clearly before you do. You might find that I have a good point.

You don't "need" to learn them, but many people and companies use them. The main advantage is being able to have your entire site on a single webpage, where you just modify the data and it handles changing the elements on the page for you. For instance, you can just think about "put the user's name here, and when they click the button, add another list field there", but you no longer have to think about "get this DOM element, set its innerHTML to the user's name, and when they click the button, create a new DOM element for input, another one for the label, get the container element, append the new children". It also allows you to more easily bind data to DOM elements, so you can just say "this field is for the variable 'numberOfItems'" instead of worrying about grabbing the DOM element and its value and storing it into numberOfItems when you need it; with React/Angular/etc. that variable will always be up to date with the user's input without you needing to update it.

TL;DR: The frameworks are not necessary, but they help automatically handle a lot of common, tedious DOM manipulation for you in a way more optimized than you'd likely end up implementing yourself, with less code for you, and all on a single page instead of having to reload the page every time you want to display new data or a new layout.

What is React JS?

ReactJS is an open-source JavaScript library that is used for building user interfaces specifically for single-page applications. It’s used for handling the view layer for web and mobile apps. React also allows us to create reusable UI components.

React allows developers to create large web applications that can change data, without reloading the page. The main purpose of React is to be fast, scalable, and simple. It works only on user interfaces in the application. This corresponds to view in the MVC template. It can be used with a combination of other JavaScript libraries or frameworks, such as Angular JS in MVC.

https://www.c-sharpcorner.com/article/what-and-why-reactjs/

This is a wide discussion about frameworks in general. Is easy to say React, Angular and so on makes easier to manipulate DOM elements and reuse components, but to apply frameworks usually are about if solve your problem.

in javascript we have load the webpage by using location.href but by using angular/react we the page updates automatically.