[jest]: TypeError: Object prototype may only be an Object or null: undefined

Question

While writing jest tests, I needed to use memfs as a mock for the Nodejs native file system module, so I used jest's manual mocks, but I'm getting this error:

> rimraf tests/{coverage,public} && jest

 PASS  tests/x.test.ts (19.926 s)
 FAIL  tests/mix.test.ts
  ● Test suite failed to run

    TypeError: Object prototype may only be an Object or null: undefined
        at Function.setPrototypeOf (<anonymous>)

      at node_modules/graceful-fs/polyfills.js:139:39
      at patch (node_modules/graceful-fs/polyfills.js:141:5)
      at patch (node_modules/graceful-fs/graceful-fs.js:104:3)
      at Object.<anonymous> (node_modules/graceful-fs/graceful-fs.js:96:18)
      at Object.<anonymous> (node_modules/fs-extra/lib/fs/index.js:5:12)

 PASS  tests/options.test.ts (35.412 s)

Test Suites: 1 failed, 2 passed, 3 total
Tests:       6 passed, 6 total
Snapshots:   0 total
Time:        36.834 s
Ran all test suites.

Here are the files for a minimal reproduction of the error:

// src/index.ts
// this is just a minimal reproduction
import "laravel-mix";
import fs from "fs";

export default function getContent(path: string) {
    return fs.readFileSync(path, "utf-8");
}

// tests/index.test.ts

import path from "path";
import fs from "fs";
import memfs from "memfs";
import getContent from "../src";

// Use memfs instead of native fs module.
jest.mock("fs");
jest.mock("fs/promises");

beforeAll(() => {
  memfs.fs.mkdirSync(path.resolve(), {recursive: true});
});

// this is for demonstration only.
test("should mock fs", () => {
    expect(fs).toBe(memfs.fs);
});

test("returns content from memfs", () => {
    memfs.fs.writeFileSync("test.txt", "test text");

    const result = getContent("test.txt");

    expect(result).toBe("test text");
});

// more tests

// jest.config.js

module.exports = {
    collectCoverageFrom: ["src/*.ts"],
    coverageDirectory: "tests/coverage",
    preset: "ts-jest",
    testEnvironment: "node",
    testMatch: ["<rootDir>/tests/**/*.test.ts"],
};

// tsconfig.json

{
  "compilerOptions": {
    "downlevelIteration": true,
    "declaration": true,
    "declarationDir": "dist",
    "strict": true,
    "esModuleInterop": true,
    "resolveJsonModule": true,
    "moduleResolution": "node",
    "sourceMap": true,
    "noImplicitReturns": true,
    "noImplicitOverride": true,
    "importHelpers": true
  }
}

// package.json

{
  "name": "jest-mock-error-reproduction",
  "version": "1.0.0",
  "description": "",
  "main": "src/index.ts",
  "scripts": {
    "test": "jest"
  },
  "keywords": [],
  "author": "",
  "license": "ISC",
  "devDependencies": {
    "@types/jest": "^27.0.1",
    "@types/node": "^16.7.8",
    "@types/serve-static": "^1.13.10",
    "jest": "^27.1.0",
    "laravel-mix": "^6.0.29",
    "memfs": "^3.2.4",
    "ts-jest": "^27.0.5",
    "typescript": "~4.2.0"
  }
}

And the manual mock files:

// __mocks__/fs.ts

import {fs} from "memfs";

export default fs;

// __mocks__/fs/promises.ts

import {fs} from "memfs";

export default fs.promises;

Note that when I remove :

jest.mock("fs");
jest.mock("fs/promises");

from tests/index.test.ts, tests fail as expected.

I tried debugging the source code that produces the error, and I couldn't find the problem.

I also tried to use import * as memfs from "memfs" syntax in the __mock__ files, because from other answers it seems like that solves the problem, but the error persists.

Any help would be appreciated.

Answer 1

I was able to narrow the source of the problem down to a dependency.

It seems like memfs is not compatible with graceful-fs which is a dependency of fs-extra which in turn is a dependency of a library I'm using in my own code (laravel-mix). So here's a very minimal reproduction of the error now:

// tests/index.test.ts

import fs from "fs";
import memfs from "memfs";
import "graceful-fs"; // comment this and the test should pass.

jest.mock("fs");
jest.mock("fs/promises");

test("should mock fs", () => {
    expect(fs).toBe(memfs.fs);
});

To solve my issue, I changed the virtual file system library in use. I switched from memfs to mock-fs.

First, I installed mock-fs and @types/mock-fs:

npm i -D mock-fs @types/mock-fs

And then I used it in tests/index.test.ts:

// tests/index.test.ts

import path from "path";
import fs from "fs";
import mock_fs from "mock-fs";
import getContent from "../src";

beforeEach(() => {
    mock_fs();
});

afterEach(() => {
    mock_fs.restore();
});

test("returns content from mocked fs", () => {
    fs.writeFileSync("test.txt", "test text");

    const result = getContent("test.txt");

    expect(result).toBe("test text");
});

// more tests