Use jest.mock and TypeScript without cluttered type casting?

Question

When I compile my a test using the TypeScript compiler and working with a Jest mock, I often receive errors from tsc like:

error TS2339: Property 'mockImplementationOnce' does not exist on type
              'typeof readFile'.

from this minimal test:

jest.mock('fs');
// Run before the imports but does not alter types :(

import { readFile } from 'fs';
import { fnThatReadsFile } from './lib';

it('should read a file', () => {
  const err = {};
  readFile.mockImplementationOnce((_, callback) => callback(err, null));
  // ^^ error TS2339: Property 'mockImplementationOnce' does not exist on type 'typeof readFile'.

  fnThatReadsFile();
  // expect...
});

What solutions are there other than:

casting: readFile as jest.Mock<{}>
module augmentation

Could a TypeScript plugin perform the module augmentation when modules are required by jest.mock?

Maybe [this method](https://stackoverflow.com/a/38414108/6392661) that someone is using helps — Dennis Skoko, Jul 18 '18 at 12:23

score 9 · Answer 1 · answered May 22 '18 at 21:08

9

simplest solution is to import fs like this: const fs = require('fs'), and use (fs.readFile as jest.Mock).mockImplementationOnce ...

answered May 22 '18 at 21:08

Herman Starikov

2,636
15
26

Thanks Herman. I've found that casting is the most verbose way to work since you need to cast it every time you're working with it. – Sean May 22 '18 at 21:42

score 3 · Answer 2 · answered Aug 20 '18 at 11:08

3

Simple solution is to import directly from the mock file. It looks inelegant, but works.

import { readFile } from '../__mocks__/fs';

answered Aug 20 '18 at 11:08

pavloko

970
9
8

Use jest.mock and TypeScript without cluttered type casting?

2 Answers2