React also let's you mark areas/divs within its domain as "don't touch this". So in those areas you can use things like that old map widget you love so much, without resorting to iframes
Angular is a huge fail in this regard. Zero support for mixing with "non angular" pages.
I'm sure experienced people know how to do that properly, but it's really difficult starting out.
Too much abstraction, too much magic. It's possible to do just about anything but the docs are sparse enough that you'll be crawling through github PR's and source to figure out how to achieve it sometimes
React's entry point is the `ReactDOM.render` function, which takes some React elements, and a root node. that root node can be _any_ DOM node (Actually no idea if you can mount React into a inline SVG, but at least any _HTML_ node you certainly can). You can also have multiple `ReactDOM.render` calls, no problem at all.
That initial React element can very well take values and callbacks from your Angular or whatever application.
It can get messy if you nest `ReactDOM.render` within roots that are managed by React, but so would nesting any other UI framework, but the use cases for that are... Exuberantly exotic, to say the least.
Usually, people have a single “mount point” very close to the <body> element, and a single script file whose primary purpose is to call ReactDOM.render to “mount” a React component representing their application at that point.
However, that doesn’t prevent you from having DOM outside that mount point, and it doesn’t prevent you from having code which does other things that don’t have anything to do with the React-managed part of the page. This is a great way to migrate a legacy application to React: you choose one small section of your page to replace with React, and then have the remainder of your page interact with it by 1.) choosing when to call ReactDOM.render again to ask it to update, and 2.) passing in callbacks so the React ”mini-app” can notify the rest of your application when something happens.
You can go the other way, too, although it’s a little less safe. If you have a React component which renders, say, a <div> with no children, you can get a handle to the DOM element itself and interact with it just like you would in a non-React application. The only constraint here is that you have to make sure to clean up e.g. event handlers and release references to those elements if something causes the “owner” React component to unmount. This is how people make wrapper libraries which let you use non-React-based libraries using React components. (I’ve done this in a side project where I had a huge number homogenous DOM elements to work with, and found that managing them myself was much more performant than trying to run them through React’s model.)