Why do ambient declarations use namespace and module?

Question

I noticed that many ambient declaration files declare a namespace and a module that merely exports the namespace, usually using some gymnastics that I don't really understand. For example, in react.d.ts you see this:

declare namespace __React {

    ... entire API of the react library ...

}

declare module "react" {
    export = __React;
}

1. Why both a namespace and a module? Why not just declare the module with the library API inside it?
2. Why is the namespace called __React and not simply React? That seems like a rather awkward "don't use me" name, yet IntelliJ IDEA has imported this all over my source code and it seems to work.

Answer 1

This pattern is used to support UMD libraries.

These libraries generally put something into the global scope if they're loaded through a <script ...> tag, but return something to a module loader if invoked via a module system like RequireJS, CommonJS, or SystemJS.

In TypeScript, this means that if you import the module called 'react', you should get the same type as if you reference the global identifier React.

Most definition files simply write their .d.ts files such that the module shape and the global variable are always both present; the React authors didn't like that you could accidently refer to the global React if you were using a module loader (in which case the global wouldn't actually be there) so they separated out the declaration into __React, a separate .d.ts file that declared a global called React, and a module declaration for "react".