Put Jest to the test by testing the appendFileSync function

Question

Put Jest to the test by testing the appendFileSync function

I am currently working on creating a test for appendfilesync function. When using a logger, I noticed that one line of code is not covered in my tests. Below is the code snippet I am referring to (please note that I am using tslog for logging purposes):

export function logToTransport(logObject: ILogObject) {
  appendFileSync('monopoly_deal.log', JSON.stringify(logObject) + '\n');
}

I have attempted to mock 'fs' in order to prevent actual file writing during testing, but this method does not allow me to properly test the writing functionality. Here is the beginning of my test setup code:

const log: Logger = new Logger({ name: 'card_types_test' });
    log.attachTransport(
      {
        silly: CardTypes.logToTransport,
        debug: CardTypes.logToTransport,
        trace: CardTypes.logToTransport,
        info: CardTypes.logToTransport,
        warn: CardTypes.logToTransport,
        error: CardTypes.logToTransport,
        fatal: CardTypes.logToTransport,
      },
      'info',
    );

// To intercept the call without executing it, I believe I need to set up jest.spyon. However, I am unsure about the correct approach. Possibly something like const spy = jest.fn(CardTypes.logToTransport);

log.info('test'); // Writes to the log file
expect(spy).toequal({object with my data}); // The specific structure of the object has been omitted for brevity.

Any advice on how to simulate file writing in my tests would be highly valuable, as well as any other suggestions or feedback (as I am still relatively new to programming).

javascript typescript unit-testing jestjs ts-jest

Answer 1

Answer №1

When testing the code logic, it is advisable to focus on the logToTransport function rather than the fs.appendFileSync method. The latter is a built-in Node.js method that is already well-tested.

If needed, you can mock the fs.appendFileSync method, also known as mocking partials, to test the inner workings of the logToTransport function. This approach aligns with white-box testing.

In addition, you can use jest.isMockFunction to confirm if the mocking partials were implemented successfully.

For example:

index.ts:

import { appendFileSync } from 'fs';

type ILogObject = any;
export function logToTransport(logObject: ILogObject) {
  appendFileSync('monopoly_deal.log', JSON.stringify(logObject) + '\n');
}

index.test.ts:

import fs from 'fs';
import { logToTransport } from '.';

jest.mock('fs', () => ({
  ...(jest.requireActual('fs') as typeof fs),
  appendFileSync: jest.fn(),
}));
describe('73466276', () => {
  test('should pass', () => {
    expect(jest.isMockFunction(fs.appendFileSync)).toBeTruthy();
    expect(jest.isMockFunction(fs.appendFile)).toBeFalsy();
    logToTransport({ level: 'debug', payload: { name: 'teresa teng' } });
    expect(fs.appendFileSync).toBeCalledWith(
      'monopoly_deal.log',
      JSON.stringify({ level: 'debug', payload: { name: 'teresa teng' } }) + '\n'
    );
  });
});

Test result:

 PASS  stackoverflow/73466276/index.test.ts (11.306 s)
  73466276
    ✓ should pass (3 ms)

----------|---------|----------|---------|---------|-------------------
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:       1 passed, 1 total
Snapshots:   0 total
Time:        12.012 s

Answer 2

When testing the code logic, it is advisable to focus on the logToTransport function rather than the fs.appendFileSync method. The latter is a built-in Node.js method that is already well-tested.

If needed, you can mock the fs.appendFileSync method, also known as mocking partials, to test the inner workings of the logToTransport function. This approach aligns with white-box testing.

In addition, you can use jest.isMockFunction to confirm if the mocking partials were implemented successfully.

For example:

index.ts:

import { appendFileSync } from 'fs';

type ILogObject = any;
export function logToTransport(logObject: ILogObject) {
  appendFileSync('monopoly_deal.log', JSON.stringify(logObject) + '\n');
}

index.test.ts:

import fs from 'fs';
import { logToTransport } from '.';

jest.mock('fs', () => ({
  ...(jest.requireActual('fs') as typeof fs),
  appendFileSync: jest.fn(),
}));
describe('73466276', () => {
  test('should pass', () => {
    expect(jest.isMockFunction(fs.appendFileSync)).toBeTruthy();
    expect(jest.isMockFunction(fs.appendFile)).toBeFalsy();
    logToTransport({ level: 'debug', payload: { name: 'teresa teng' } });
    expect(fs.appendFileSync).toBeCalledWith(
      'monopoly_deal.log',
      JSON.stringify({ level: 'debug', payload: { name: 'teresa teng' } }) + '\n'
    );
  });
});

Test result:

 PASS  stackoverflow/73466276/index.test.ts (11.306 s)
  73466276
    ✓ should pass (3 ms)

----------|---------|----------|---------|---------|-------------------
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:       1 passed, 1 total
Snapshots:   0 total
Time:        12.012 s

Put Jest to the test by testing the appendFileSync function

Answer №1

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