Mocking module method in Jest while keeping original TypeScript typing

Igor Rogatty

2022-05-30

jest, typescript

For some time I was ignoring PhpStorm’s warning about missing types for the uuid npm module. Today I decided to use a quick-action to install @types/uuid.

It made IDE happy but caused type issues for the following (simplified) code in tests:

1 2	import { v4 } from "uuid"; v4.mockReturnValue(123);

TypeScript was throwing error:

1	error TS2339: Property 'mockReturnValue' does not exist on type 'v4'.

I found https://stackoverflow.com/a/60300568/1050577 which is a great answer for this issue:

1	(v4 as jest.MockedFunction<typeof v4>).mockReturnValue("123");

I like that I no longer see the warning in IDE. But even better, TypeScript noticed that mocking uuid.v4 return value as a number is not compatible with its original implementation. Fixing this made the tests closer to the reality, which in turn made the app more robust.