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I have handlers for unhandledRejections and uncaughtExceptions:

bin.js

['unhandledRejection', 'uncaughtException'].forEach(event => {
  process.on(event, err => logger.error(err));
});

Now I want to test them with jest:

bin.test.js

const bin = require('../bin');

test('catches unhandled rejections', async () => {
  const error = new Error('mock error');
  await Promise.reject(error);
  expect(logger.error).toHaveBeenCalledWith(error);
});

test('catches uncaught exceptions', () => {
  const error = new Error('mock error');
  throw error;
  expect(logger.error).toHaveBeenCalledWith(error);
});

But jest just tells me that there are errors in the tests:

● catches unhandled rejections

mock error

   8 | // 
   9 | test('catches unhandled rejections', async () => {
> 10 |   const error = new Error('mock error');
     |                 ^
  11 |   await Promise.reject(error);
  12 |   expect(logger.error).toHaveBeenCalledWith(error);
  13 | });

  at Object.<anonymous>.test (test/bin.test.js:10:17)

● catches uncaught exceptions

mock error

  14 |
  15 | test('catches uncaught exceptions', () => {
> 16 |   const error = new Error('mock error');
     |                 ^
  17 |   throw error;
  18 |   expect(logger.error).toHaveBeenCalledWith(error);
  19 | });

  at Object.<anonymous>.test (test/bin.test.js:16:17)

is there a way to test this?

This might be related:

I have handlers for unhandledRejections and uncaughtExceptions:

bin.js

['unhandledRejection', 'uncaughtException'].forEach(event => {
  process.on(event, err => logger.error(err));
});

Now I want to test them with jest:

bin.test.js

const bin = require('../bin');

test('catches unhandled rejections', async () => {
  const error = new Error('mock error');
  await Promise.reject(error);
  expect(logger.error).toHaveBeenCalledWith(error);
});

test('catches uncaught exceptions', () => {
  const error = new Error('mock error');
  throw error;
  expect(logger.error).toHaveBeenCalledWith(error);
});

But jest just tells me that there are errors in the tests:

● catches unhandled rejections

mock error

   8 | // https://github./facebook/jest/issues/5620
   9 | test('catches unhandled rejections', async () => {
> 10 |   const error = new Error('mock error');
     |                 ^
  11 |   await Promise.reject(error);
  12 |   expect(logger.error).toHaveBeenCalledWith(error);
  13 | });

  at Object.<anonymous>.test (test/bin.test.js:10:17)

● catches uncaught exceptions

mock error

  14 |
  15 | test('catches uncaught exceptions', () => {
> 16 |   const error = new Error('mock error');
     |                 ^
  17 |   throw error;
  18 |   expect(logger.error).toHaveBeenCalledWith(error);
  19 | });

  at Object.<anonymous>.test (test/bin.test.js:16:17)

is there a way to test this?

This might be related: https://github./facebook/jest/issues/5620

• javascript
• node.js
• unit-testing
• jestjs

Share Improve this question Follow edited Dec 21, 2020 at 5:00 Lin Du 103k136136 gold badges334334 silver badges566566 bronze badges asked Sep 25, 2018 at 7:48 LukasLukas 10.4k1717 gold badges8282 silver badges128128 bronze badges 2

• I think this is doable but I need to know how logger gets built – jcollum Commented Sep 27, 2018 at 21:31
• 1 @jcollum like this const logger = winston.createLogger({/* ...some options */}); – Lukas Commented Oct 2, 2018 at 13:21

Add a ment  | 

2 Answers 2

Sorted by: Reset to default 5

My test strategy is to install spy onto process.on() and logger.error methods using jest.spyOn(object, methodName). After doing this, these methods have no side effects. Then, you can test your code logic in an isolated environment.

Besides, there are a few things to note:

• You should spy the functions before require('./bin') statement. Because when you load the bin.js module, the code will be executed.
• You should use jest.resetModules() in the beforeEach hook to resets the module registry - the cache of all required modules. Why? because require() caches its results. So, the first time a module is required, then its initialization code runs. After that, the cache just returns the value of module.exports without running the initialization code again. But we have two test cases, we want the code in module scope to be executed twice.

Now, here is the example:

bin.js:

const logger = require('./logger');

['unhandledRejection', 'uncaughtException'].forEach((event) => {
  process.on(event, (err) => logger.error(err));
});

logger.js:

const logger = console;

module.exports = logger;

bin.test.js:

const logger = require('./logger');

describe('52493145', () => {
  beforeEach(() => {
    jest.resetModules();
  });
  afterEach(() => {
    jest.restoreAllMocks();
  });
  test('catches unhandled rejections', () => {
    const error = new Error('mock error');
    jest.spyOn(process, 'on').mockImplementation((event, handler) => {
      if (event === 'unhandledRejection') {
        handler(error);
      }
    });
    jest.spyOn(logger, 'error').mockReturnValueOnce();
    require('./bin');
    expect(process.on).toBeCalledWith('unhandledRejection', expect.any(Function));
    expect(logger.error).toHaveBeenCalledWith(error);
  });

  test('catches uncaught exceptions', () => {
    const error = new Error('mock error');
    jest.spyOn(process, 'on').mockImplementation((event, handler) => {
      if (event === 'uncaughtException') {
        handler(error);
      }
    });
    jest.spyOn(logger, 'error').mockReturnValueOnce();
    require('./bin');
    expect(process.on).toBeCalledWith('uncaughtException', expect.any(Function));
    expect(logger.error).toHaveBeenCalledWith(error);
  });
});

unit test result:

 PASS  examples/52493145/bin.test.js
  52493145
    ✓ catches unhandled rejections (5 ms)
    ✓ catches uncaught exceptions (1 ms)

-----------|---------|----------|---------|---------|-------------------
File       | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s 
-----------|---------|----------|---------|---------|-------------------
All files  |     100 |      100 |     100 |     100 |                   
 bin.js    |     100 |      100 |     100 |     100 |                   
 logger.js |     100 |      100 |     100 |     100 |                   
-----------|---------|----------|---------|---------|-------------------
Test Suites: 1 passed, 1 total
Tests:       2 passed, 2 total
Snapshots:   0 total
Time:        2.73 s, estimated 4 s

source code: https://github./mrdulin/jest-v26-codelab/tree/main/examples/52493145

putting it inside try catch will help:

const error = new Error('mock error');

try {

await Promise.reject(error);

} catch(error){

   expect(logger.error).toHaveBeenCalledWith(error);

}

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