I have a module that uses a second module. They're both custom modules, not NPM packages.
I want to make sure that moduleUnderTest calls a specific method from foo.
So I'm using jest.mock and I pass it a function that has the same signature as foo but contains jest spy functions instead of the actual implementation.
I was under the impression that using jest.mock with the mockFoo object would inject the mocked foo instead of the actual module when moduleUnderTest requires foo.
If I inspect what foo is from within moduleUnderTest while the test is running, I can see that it is indeed my mocked foo.
But when I get to the expect, mockedFoo.met1 bees undefined.
Why is that?
// ../foo/foo.js
const foo = arg => {
console.log(arg)
return {
met1: () => {},
met2: () => {},
}
}
module.exports = foo
// ..foo/index.js
// I am doing it this way so I can put the actual implementation in
// several different files, but can require the whole module with
// require('../foo') rather than require('../foo/foo')
module.exports = require('./foo')
// ./moduleUnderTest.js
const foo = require('../foo')('hi')
require('util').inspect(foo) // it seems that foo is indeed the mocked
// version here[0]
const moduleUnderTest = () => {
foo.met1()
}
module.exports = moduleUnderTest
// ./moduleUnderTest.test.js
const moduleUnderTest = require('./moduleUnderTest')
const mockFoo = () => ({
met1: jest.fn(),
met2: jest.fn(),
})
jest.mock('../foo')
test('foo.met1 is called', () => {
moduleUnderTest()
expect(mockFoo.met1).toHaveBeenCalledTimes(1) // NOPE![1]
})
// [0]
// { met1:
// { [Function: mockConstructor]
// _isMockFunction: true,
// getMockImplementation: [Function],
// mock: [Getter/Setter],
// mockClear: [Function],
// mockReset: [Function],
// mockReturnValueOnce: [Function],
// mockResolvedValueOnce: [Function],
// mockRejectedValueOnce: [Function],
// mockReturnValue: [Function],
// mockResolvedValue: [Function],
// mockRejectedValue: [Function],
// mockImplementationOnce: [Function],
// mockImplementation: [Function],
// mockReturnThis: [Function],
// mockName: [Function],
// getMockName: [Function],
// mockRestore: [Function] },
// met2:
// { [Function: mockConstructor]
// _isMockFunction: true,
// getMockImplementation: [Function],
// mock: [Getter/Setter],
// mockClear: [Function],
// mockReset: [Function],
// mockReturnValueOnce: [Function],
// mockResolvedValueOnce: [Function],
// mockRejectedValueOnce: [Function],
// mockReturnValue: [Function],
// mockResolvedValue: [Function],
// mockRejectedValue: [Function],
// mockImplementationOnce: [Function],
// mockImplementation: [Function],
// mockReturnThis: [Function],
// mockName: [Function],
// getMockName: [Function],
// mockRestore: [Function] } }
// [1]
// expect(jest.fn())[.not].toHaveBeenCalledTimes()
//
// jest.fn() value must be a mock function or spy.
// Received: undefined
I have a module that uses a second module. They're both custom modules, not NPM packages.
I want to make sure that moduleUnderTest calls a specific method from foo.
So I'm using jest.mock and I pass it a function that has the same signature as foo but contains jest spy functions instead of the actual implementation.
I was under the impression that using jest.mock with the mockFoo object would inject the mocked foo instead of the actual module when moduleUnderTest requires foo.
If I inspect what foo is from within moduleUnderTest while the test is running, I can see that it is indeed my mocked foo.
But when I get to the expect, mockedFoo.met1 bees undefined.
Why is that?
// ../foo/foo.js
const foo = arg => {
console.log(arg)
return {
met1: () => {},
met2: () => {},
}
}
module.exports = foo
// ..foo/index.js
// I am doing it this way so I can put the actual implementation in
// several different files, but can require the whole module with
// require('../foo') rather than require('../foo/foo')
module.exports = require('./foo')
// ./moduleUnderTest.js
const foo = require('../foo')('hi')
require('util').inspect(foo) // it seems that foo is indeed the mocked
// version here[0]
const moduleUnderTest = () => {
foo.met1()
}
module.exports = moduleUnderTest
// ./moduleUnderTest.test.js
const moduleUnderTest = require('./moduleUnderTest')
const mockFoo = () => ({
met1: jest.fn(),
met2: jest.fn(),
})
jest.mock('../foo')
test('foo.met1 is called', () => {
moduleUnderTest()
expect(mockFoo.met1).toHaveBeenCalledTimes(1) // NOPE![1]
})
// [0]
// { met1:
// { [Function: mockConstructor]
// _isMockFunction: true,
// getMockImplementation: [Function],
// mock: [Getter/Setter],
// mockClear: [Function],
// mockReset: [Function],
// mockReturnValueOnce: [Function],
// mockResolvedValueOnce: [Function],
// mockRejectedValueOnce: [Function],
// mockReturnValue: [Function],
// mockResolvedValue: [Function],
// mockRejectedValue: [Function],
// mockImplementationOnce: [Function],
// mockImplementation: [Function],
// mockReturnThis: [Function],
// mockName: [Function],
// getMockName: [Function],
// mockRestore: [Function] },
// met2:
// { [Function: mockConstructor]
// _isMockFunction: true,
// getMockImplementation: [Function],
// mock: [Getter/Setter],
// mockClear: [Function],
// mockReset: [Function],
// mockReturnValueOnce: [Function],
// mockResolvedValueOnce: [Function],
// mockRejectedValueOnce: [Function],
// mockReturnValue: [Function],
// mockResolvedValue: [Function],
// mockRejectedValue: [Function],
// mockImplementationOnce: [Function],
// mockImplementation: [Function],
// mockReturnThis: [Function],
// mockName: [Function],
// getMockName: [Function],
// mockRestore: [Function] } }
// [1]
// expect(jest.fn())[.not].toHaveBeenCalledTimes()
//
// jest.fn() value must be a mock function or spy.
// Received: undefined
1 Answer
Reset to default 2I found what the issue is.
Because foo is instantiated by executing the function, calling mockFoo.met1 returns undefined since mockFoo is a function. For mock1 to be defined, I'd have to call mockFoo().met1. But at that point I'm getting a new instance of mockFoo which is different from the one loaded and called by moduleUnderTest.
To fix the problem, I have to declare and set the mock this way:
const mockMet1 = jest.fn()
const mockMet2 = jest.fn()
const mockFoo = () => ({
met1: mockMet1,
met2: mockMet2,
})
const moduleUnderTest = require('./moduleUnderTest')
jest.mock('../foo', mockFoo)
// ...snip...
afterEach(() => { // resets the called count etc
mockMet1.resetMock()
mockMet2.resetMock()
})
// ...snip...
expect(mockMet1).toHaveBeenCalledTimes(1)
// ...snip...