Or you can configure the TS compiler to allow JS imports, then everything also compiles and works, but you can slowly convert your codebase from JS to TS file b

Or you can configure the TS compiler to allow JS imports, then everything also compiles and works, but you can slowly convert your codebase from JS to TS file by file and be sure that all TS files are properly typed and all JS files are untyped instead of having everything as TS files where some are typed and some are not.