Reputation: 951
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.
Upvotes: 4
Views: 4499
Reputation: 951
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
Upvotes: 4