I have the following hypothetical scenario:
// file MyClass.js in an external package
class MyClass {
myfunc = () => {
// do something
}
}
// file in my project
function myFunctionToBeTested() {
const instance = new MyClass()
instance.myFunc()
}
I need to create a test with Jest that makes sure instance.myFunc was called
One of the option is to replace MyClass module with mock implementation
const mockmyfunc = jest.fn()
jest.mock("path/to/external/package/MyClass", () => {
return jest.fn().mockImplementation(() => {
return {myfunc: mockmyfunc}
})
})
And then write following test
it("Test myfunc called in functionToBeTested", () => {
functionToBeTested()
expect(mockmyfunc).toHaveBeenCalled()
})
Note that this is not the only way, you can dive into https://facebook.github.io/jest/docs/en/es6-class-mocks.html for other alternatives.
If the myfunc would be an actual function (which i guess is not an option since it's external package?)
export class MyClass {
myFunc() {
// do smth
}
}
and you would not need to replace the implementation, you could be using jest's automock
import MyClass from "path/to/external/package/MyClass"
jest.mock("path/to/external/package/MyClass")
it("Test myfunc called in functionToBeTested", () => {
functionToBeTested()
const mockMyFunc = MyClass.mock.instances[0].myFunc
expect(mockMyFunc).toHaveBeenCalled()
})
you can mock out the class and assign the default export of that file to a variable as follows:
jest.mock('../../utils/api/api');
const FakeClass = require('../someFile.js').default;
then access calls to a function on your mock class like this:
FakeClass.prototype.myFunc.mock.calls
