Babel hoist TypeScript issue repro

This is (in my opinion) a reasonably configured TypeScript project that uses Jest.

Config

It sets the following relevant TypeScript configuration (see tsconfig.json):

• noImplicitAny: true (to avoid buggy untyped code)
• allowSyntheticDefaultImports: true (to support ES-style exports)

We have the bare minimum of Babel config, which loads @babel/preset-env targeting node, and @babel/preset-typescript (see .babelrc).

There is no Jest configuration, so we're seeing the default behaviour that is

1. Read .babelrc, and apply specified presets/plugins and babel-plugin-jest-hoist
2. Perform the test run

Test subject

The test subject is a module that exports an Express middleware which performs some work that is undesirable in a test run. In this case, it does console.log('hi'). My real case is a request logger that gets incredibly noisy when performing tests that send HTTP requests to the server.

Test case

I want to mock the middleware exported from the test subject. Express (or Connect, if you prefer) requires a call to the next function in order to move on to the next middleware, so I can't just let Jest provide a mock function; I need to provide a fake middleware in place of the original one.

Untyped solution

Jest is happy with this:

jest.mock('./index', () => {
  const fakeMiddleware = (_req, _res, next) => next();

  return {
    middleware: fakeMiddleware,
  };
});

test('1+1', () => {
  expect(1 + 1).toBe(2);
});

TypeScript, however, yells at me because _req, _res, and next are all untyped, and thus fail the noImplicitAny check:

Typed solution

I can explicitly type fakeMiddleware as an express.RequestHandler like this:

import { RequestHandler } from 'express';

jest.mock('./index', () => {
  const fakeMiddleware: RequestHandler = (_req, _res, next) => next();

  return {
    middleware: fakeMiddleware,
  };
});

test('1+1', () => {
  expect(1 + 1).toBe(2);
});

This makes TypeScript happy. jest, on the other hand, is not:

The problem

Since Jest does not perform any checks related to typings, it makes sense to me that type information should be stripped before mocks are hoisted. As Jest claims I reference an out-of-scope "variable" (actually a type), this does not seem to be the case. To me (and @simenb, who looked at this after I gave up), this seems like a bug.