jest_mock_imported_constant.js

//------------------------------------------------
// index.js
//-----------------------------------------------

// Import method to call the external service
const {callExternalService} = require('./externalService');
// Method to test
const main = () => {
  // We only call the external service and return its response
  return callExternalService();
}

module.exports = {main};

//------------------------------------------------
// externalService.js
//------------------------------------------------

// Some external call to a service, returning some result
const callExternalService = () => {
  // Do some logic/API calls here...
  return true;
}

module.exports = {callExternalService};

//------------------------------------------------
// index.spec.js
//------------------------------------------------

const {main} = require('./index');
// Import the external call we want to 
// return a custom value for
const {callExternalService} = require('./externalService');

// Mock the whole external services module
jest.mock('./externalService');

describe('index', () => {

  beforeEach(() => {
    // Resets the overwritten return values via
    // "callExternalService.mockReturnValue" in the test cases
    jest.resetAllMocks();
  });

  it('Should test success case', () => {
    // We test the case where the external service
    // returns "true"
    callExternalService.mockReturnValue(true)
    
    // Call the main() method and expect that it
    // return "true" after processing the mocked response
    // of our external service call
    const result = main();
    expect(result).toBe(true);
  });

  it('Should test failure case', () => {
    // We test the case where the external service
    // returns "false"
    callExternalService.mockReturnValue(false)
    // Call the main() method and expect that it
    // return "false" after processing the mocked response
    // of our external service call 
    const result = main();
    expect(result).toBe(false);
  });

});