Previously I have worked on a solution that comprised of completely Typescript files. In this currently solution there are still JavaScript files waiting to be moved over.

Javascript global variables will work perfectly fine in Typescript but you loose the important class typing. Like any other language Typescript has a casting function the compiler can comprehend. Those working completely with Typescript may never have to use this but those with a fragmented solution can use the "as" keyword (recommended over as it conflicts with React components) to handle their casting.