How to change mock implementation on a per single test basis [Jestjs]
Solution 1
A nice pattern for writing tests is to create a setup factory function that returns the data you need for testing the current module.
Below is some sample code following your second example although allows the provision of default and override values in a reusable way.
const spyReturns = returnValue => jest.fn(() => returnValue);
describe("scenario", () => {
beforeEach(() => {
jest.resetModules();
});
const setup = (mockOverrides) => {
const mockedFunctions = {
a: spyReturns(true),
b: spyReturns(true),
...mockOverrides
}
jest.doMock('../myModule', () => mockedFunctions)
return {
mockedModule: require('../myModule')
}
}
it("should return true for module a", () => {
const { mockedModule } = setup();
expect(mockedModule.a()).toEqual(true)
});
it("should return override for module a", () => {
const EXPECTED_VALUE = "override"
const { mockedModule } = setup({ a: spyReturns(EXPECTED_VALUE)});
expect(mockedModule.a()).toEqual(EXPECTED_VALUE)
});
});
It's important to say that you must reset modules that have been cached using jest.resetModules()
. This can be done in beforeEach
or a similar teardown function.
See jest object documentation for more info: https://jestjs.io/docs/jest-object.
Solution 2
Use mockFn.mockImplementation(fn).
import { funcToMock } from './somewhere';
jest.mock('./somewhere');
beforeEach(() => {
funcToMock.mockImplementation(() => { /* default implementation */ });
// (funcToMock as jest.Mock)... in TS
});
test('case that needs a different implementation of funcToMock', () => {
funcToMock.mockImplementation(() => { /* implementation specific to this test */ });
// (funcToMock as jest.Mock)... in TS
// ...
});
Solution 3
Little late to the party, but if someone else is having issues with this.
We use TypeScript, ES6 and babel for react-native development.
We usually mock external NPM modules in the root __mocks__
directory.
I wanted to override a specific function of a module in the Auth class of aws-amplify for a specific test.
import { Auth } from 'aws-amplify';
import GetJwtToken from './GetJwtToken';
...
it('When idToken should return "123"', async () => {
const spy = jest.spyOn(Auth, 'currentSession').mockImplementation(() => ({
getIdToken: () => ({
getJwtToken: () => '123',
}),
}));
const result = await GetJwtToken();
expect(result).toBe('123');
spy.mockRestore();
});
Gist: https://gist.github.com/thomashagstrom/e5bffe6c3e3acec592201b6892226af2
Tutorial: https://medium.com/p/b4ac52a005d#19c5
Solution 4
When mocking a single method (when it's required to leave the rest of a class/module implementation intact) I discovered the following approach to be helpful to reset any implementation tweaks from individual tests.
I found this approach to be the concisest one, with no need to jest.mock
something at the beginning of the file etc. You need just the code you see below to mock MyClass.methodName
. Another advantage is that by default spyOn
keeps the original method implementation but also saves all the stats (# of calls, arguments, results etc.) to test against, and keeping the default implementation is a must in some cases. So you have the flexibility to keep the default implementation or to change it with a simple addition of .mockImplementation
as mentioned in the code below.
The code is in Typescript with comments highlighting the difference for JS (the difference is in one line, to be precise). Tested with Jest 26.6.
describe('test set', () => {
let mockedFn: jest.SpyInstance<void>; // void is the return value of the mocked function, change as necessary
// For plain JS use just: let mockedFn;
beforeEach(() => {
mockedFn = jest.spyOn(MyClass.prototype, 'methodName');
// Use the following instead if you need not to just spy but also to replace the default method implementation:
// mockedFn = jest.spyOn(MyClass.prototype, 'methodName').mockImplementation(() => {/*custom implementation*/});
});
afterEach(() => {
// Reset to the original method implementation (non-mocked) and clear all the mock data
mockedFn.mockRestore();
});
it('does first thing', () => {
/* Test with the default mock implementation */
});
it('does second thing', () => {
mockedFn.mockImplementation(() => {/*custom implementation just for this test*/});
/* Test utilising this custom mock implementation. It is reset after the test. */
});
it('does third thing', () => {
/* Another test with the default mock implementation */
});
});
Solution 5
I did not manage to define the mock inside the test itself so I discover that I could mock several results for the same service mock like this :
jest.mock("@/services/ApiService", () => {
return {
apiService: {
get: jest.fn()
.mockResolvedValueOnce({response: {value:"Value", label:"Test"}})
.mockResolvedValueOnce(null),
}
};
});
I hope it'll help someone :)
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Comments
-
Andrea Carraro about 2 years
I'd like to change the implementation of a mocked dependency on a per single test basis by extending the default mock's behaviour and reverting it back to the original implementation when the next test executes.
More briefly this is what I'm trying to achieve:
- mock dependency
- change/extend mock implementation in a single test
- revert back to original mock when next test executes
I'm currently using
Jest v21
.Here is what a typical Jest test would look like:
__mocks__/myModule.js
const myMockedModule = jest.genMockFromModule('../myModule'); myMockedModule.a = jest.fn(() => true); myMockedModule.b = jest.fn(() => true); export default myMockedModule;
__tests__/myTest.js
import myMockedModule from '../myModule'; // Mock myModule jest.mock('../myModule'); beforeEach(() => { jest.clearAllMocks(); }); describe('MyTest', () => { it('should test with default mock', () => { myMockedModule.a(); // === true myMockedModule.b(); // === true }); it('should override myMockedModule.b mock result (and leave the other methods untouched)', () => { // Extend change mock myMockedModule.a(); // === true myMockedModule.b(); // === 'overridden' // Restore mock to original implementation with no side effects }); it('should revert back to default myMockedModule mock', () => { myMockedModule.a(); // === true myMockedModule.b(); // === true }); });
Here is what I've tried so far:
1 - mockFn.mockImplementationOnce(fn)
pros
- Reverts back to original implementation after first call
cons
- It breaks if the test calls
b
multiple times - It doesn't revert to original implementation until
b
is not called (leaking out in the next test)
code:
it('should override myModule.b mock result (and leave the other methods untouched)', () => { myMockedModule.b.mockImplementationOnce(() => 'overridden'); myModule.a(); // === true myModule.b(); // === 'overridden' });
2 - jest.doMock(moduleName, factory, options)
pros
- Explicitly re-mocks on every test
cons
- Cannot define default mock implementation for all tests
- Cannot extend default implementation forcing to re-declare each mocked method
code:
it('should override myModule.b mock result (and leave the other methods untouched)', () => { jest.doMock('../myModule', () => { return { a: jest.fn(() => true, b: jest.fn(() => 'overridden', } }); myModule.a(); // === true myModule.b(); // === 'overridden' });
3 - Manual mocking with setter methods (as explained here)
pros
- Full control over mocked results
cons
- Lot of boilerplate code
- Hard to maintain on long term
code:
__mocks__/myModule.js
const myMockedModule = jest.genMockFromModule('../myModule'); let a = true; let b = true; myMockedModule.a = jest.fn(() => a); myMockedModule.b = jest.fn(() => b); myMockedModule.__setA = (value) => { a = value }; myMockedModule.__setB = (value) => { b = value }; myMockedModule.__reset = () => { a = true; b = true; }; export default myMockedModule;
__tests__/myTest.js
it('should override myModule.b mock result (and leave the other methods untouched)', () => { myModule.__setB('overridden'); myModule.a(); // === true myModule.b(); // === 'overridden' myModule.__reset(); });
4 - jest.spyOn(object, methodName)
cons
- I can't revert back
mockImplementation
to the original mocked return value, therefore affecting the next tests
code:
beforeEach(() => { jest.clearAllMocks(); jest.restoreAllMocks(); }); // Mock myModule jest.mock('../myModule'); it('should override myModule.b mock result (and leave the other methods untouched)', () => { const spy = jest.spyOn(myMockedModule, 'b').mockImplementation(() => 'overridden'); myMockedModule.a(); // === true myMockedModule.b(); // === 'overridden' // How to get back to original mocked value? });
-
mrbinky3000 about 5 yearsNice. But how do you do option 2 for a npm module like '@private-repo/module'? Most examples I see have relative paths? Does this work for installed modules as well?
-
Eddy Vinck about 3 yearsThis was the only thing that worked for me, with the least amount of boilerplate. In my scenario I had a named export in TypeScript from a package without a default export, so I ended up using
import * as MyModule;
and thenconst { useQuery } = MyModule
so I could still use the imports the same way without doingMyModule.someExport
everywhere. -
ronnyrr almost 3 yearsThis is actually not working for me. In your case
mockedModule
is returningmockedModule: typeof jest
where.a()
isundefined
. Instead returning things likeadvanceTimersByTime
,clearMocks
,resetAllMocks
etc.. -
Doug Wilhelm over 2 yearsThis worked for me when mocking
date-fns-tz
'sformat
function. -
Max over 2 years@ronnyrr, true, you must use
require(...)
to obtain mocked module after callingjest.doMock()
. Also, you must calljest.resetModules()
insidebeforeEach()
so that your subsequentrequire(...)
calls use the "latest" mocked version of a module. I've submitted an edit about that to the answer to show that in the code too. -
Carson Wood over 2 yearsthis should be the accepted answer
-
utkarsh over 2 yearsmedium article mentioned here is gold :)
-
TemporaryFix about 2 yearsI don't like this answer because it requires duplicating the logic of the function you want to mock. If you change the source function you now have to remember to update every test class that mocks its implementation with its actual logic.