是否可以为 Node.js fs.readFile() 编写 Jest 单元测试？

Question

I am new to Jest unit testing and was wondering if Jest can be used to test Node.js file system modules.

I currently have a text file that contains a short poem, and the viewText function console logs the poem on my terminal.

Using Jest, I my goal is to write a test that checks if viewText function is indeed working.

const viewText = () => {
  fs.readFile('poem.txt', 'utf8', (err, data) => {
    if (err) throw err;
    console.log(data);
  });
};

With Jest, I have tried:

jest.spyOn(global.console, 'log');

const mockPoem = 'Some say the world will end in fire, Some say in ice. From what I’ve tasted of desire I hold with those who favor fire ... And would suffice.';

describe('viewText', () => {
  const mockReadFile = jest.fn();
  mockReadFile.mockReturnValue(mockPoem);

  it('prints poem to console', () => {
    viewText();
    expect(global.console.log).toHaveBeenCalledWith(mockPoem);
  });
});

With the test, all I want to do check if my viewText function passes the test - be able to view the mockPoem . I'm really confused as to how I should approach writing unit tests for functions using the file system module.

Answer 1

Here is an UT solution:

index.ts :

import fs from 'fs';

export const viewText = () => {
  fs.readFile('poem.txt', 'utf8', (err, data) => {
    if (err) throw err;
    console.log(data);
  });
};

index.spec.ts :

import { viewText } from './';
import fs from 'fs';

const mockPoem =
  'Some say the world will end in fire, Some say in ice. From what I’ve tasted of desire I hold with those who favor fire ... And would suffice.';

describe('viewText', () => {
  afterEach(() => {
    jest.restoreAllMocks();
  });
  test('prints poem to console', done => {
    const logSpy = jest.spyOn(console, 'log');
    let readFileCallback;
    // @ts-ignore
    jest.spyOn(fs, 'readFile').mockImplementation((path, options, callback) => {
      readFileCallback = callback;
    });

    viewText();
    readFileCallback(null, mockPoem);
    expect(logSpy).toBeCalledWith(mockPoem);
    expect(fs.readFile).toBeCalledWith('poem.txt', 'utf8', readFileCallback);
    done();
  });

  test('should throw error when read file failed', done => {
    let readFileCallback;
    // @ts-ignore
    jest.spyOn(fs, 'readFile').mockImplementation((path, options, callback) => {
      readFileCallback = callback;
    });

    viewText();
    const mError = new Error('read file failed');
    expect(() => readFileCallback(mError, null)).toThrowError(mError);
    expect(fs.readFile).toBeCalledWith('poem.txt', 'utf8', readFileCallback);
    done();
  });
});

Unit test result with 100% coverage:

 PASS  src/stackoverflow/58810079/index.spec.ts (11.118s)
  viewText
    ✓ prints poem to console (23ms)
    ✓ should throw error when read file failed (3ms)

  console.log node_modules/jest-mock/build/index.js:860
    Some say the world will end in fire, Some say in ice. From what I’ve tasted of desire I hold with those who favor fire ... And would suffice.

----------|----------|----------|----------|----------|-------------------|
File      |  % Stmts | % Branch |  % Funcs |  % Lines | Uncovered Line #s |
----------|----------|----------|----------|----------|-------------------|
All files |      100 |      100 |      100 |      100 |                   |
 index.ts |      100 |      100 |      100 |      100 |                   |
----------|----------|----------|----------|----------|-------------------|
Test Suites: 1 passed, 1 total
Tests:       2 passed, 2 total
Snapshots:   0 total
Time:        13.129s

Source code: https://github.com/mrdulin/jest-codelab/tree/master/src/stackoverflow/58810079