Simulating a winston logger in jest testing

Question

Simulating a winston logger in jest testing

I am pondering how to create mock transports for the File module within the Winston node package. While utilizing Jest, the __mocks__/winston.ts file is automatically loaded. My dilemma lies in the fact that I am unable to mock it due to the presence of the new keyword.

// LoggerFactory.ts
import { transports, TransportInstance } from "winston";

...
    const transportList: TransportInstance[] = [
        new transports.File({
            name: `${tag}-error`,
            filename: `${dirname}${filename}.error.log`,
            json: false,
            level: "error",
            zippedArchive: false,
            maxFiles: 14,
            maxsize: 100000000
        }),
        new transports.File({
            name: `${tag}-info`,
            filename: `${dirname}${filename}.log`,
            json: false,
            level: "info",
            maxFiles: 10,
            zippedArchive: false,
            maxsize: 100000000
        })
    ];
...

// __mocks__/winston.ts
const winston = {
    ????
};
export default winston;

An error has surfaced: TypeError: Cannot read property 'File' of undefined

javascript typescript mocking jestjs winston

Answer 1

Answer №1

When creating mocks for our winston testing in the file __mocks__/winston.js, we follow this structure:

const logger = {
  format: {
    printf: jest.fn(),
    timestamp: jest.fn(),
    simple: jest.fn(),
    colorize: jest.fn(),
    combine: jest.fn()
  },
  transports: {
    Console: jest.fn(),
    File: jest.fn()
  },
  createLogger: jest.fn().mockImplementation(function(creationOpts) {
    return {
      info: jest.fn(),
      warn: jest.fn(),
      error: jest.fn()
    };
  })
};

module.exports = logger;

By using the jest.fn() catchers, we are able to test the logger functionality without creating log files during the testing process, as logging is typically not the main focus of our tests.

Answer 2

When creating mocks for our winston testing in the file __mocks__/winston.js, we follow this structure:

const logger = {
  format: {
    printf: jest.fn(),
    timestamp: jest.fn(),
    simple: jest.fn(),
    colorize: jest.fn(),
    combine: jest.fn()
  },
  transports: {
    Console: jest.fn(),
    File: jest.fn()
  },
  createLogger: jest.fn().mockImplementation(function(creationOpts) {
    return {
      info: jest.fn(),
      warn: jest.fn(),
      error: jest.fn()
    };
  })
};

module.exports = logger;

By using the jest.fn() catchers, we are able to test the logger functionality without creating log files during the testing process, as logging is typically not the main focus of our tests.

Answer 3

Answer №2

Make sure to include the following code at the beginning of your test file:

const mockWinstonLog = jest.fn()
jest.mock('winston', () => ({
  createLogger: jest.fn(() => ({
    log: mockWinstonLog,
  })),
  transports: {
    Console: jest.fn(),
  },
  format: {
    json: jest.fn(),
  },
}))

For your test scenarios, verify the following:

expect(mockWinstonLog).toHaveBeenCalled()

Answer 4

Make sure to include the following code at the beginning of your test file:

const mockWinstonLog = jest.fn()
jest.mock('winston', () => ({
  createLogger: jest.fn(() => ({
    log: mockWinstonLog,
  })),
  transports: {
    Console: jest.fn(),
  },
  format: {
    json: jest.fn(),
  },
}))

For your test scenarios, verify the following:

expect(mockWinstonLog).toHaveBeenCalled()

Simulating a winston logger in jest testing

Answer №1

Answer №2

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