Jest with TypeScript
Problem with Jest mocks and Typescript
Mocking functions happens at runtime. At compile-time however, the Typescript compiler doesn't know that you have mocked those functions. During compilation, the TSC will do a type-check and it will complain that mockReturnValueOnce does not exist on that function.
- To solve this, we have to extend our function so it both have the original types and the type of a Jest Mock function:
jest.mock("../models/events");
import { getEvents } from "../models/events";
const mockedGetEvents = getEvents as jest.MockedFunction<typeof getEvents>
ts-jest
ts-jest is a jest transformer so that we can transform our source Typescript code into Javascript code for the purposes of our test.
API
mocked(module, deep)
The purpose of this function is to resolve typing errors
- when we declare:
const mockedFoo = mocked(foo, true)
all of the types within the mockedFoo module will be known to our jest test, and we will get no typing errors as a result.