It looks as though the lovely folks behind diff now include types in their package. Relatedly, it looks like DefinitelyTyped has removed the types package for diff as a result. So when going through this lesson, you end up getting the initial error instructing you to Try npm i --save-dev @types/diffif it exists or add a new declaration (.d.ts) file containingdeclare module 'diff';`, but that doesn't actually solve the problem, since you then end up with a @types folder with a README that says:
This is a stub types definition for @types/diff (undefined).
diff provides its own type definitions, so you don't need @types/diff installed!