I'm attempting to mock the Uppy class from the uppy package in a Jest unit test following guidance from this previously answered question. My failing code is below:
import Uppy from '@uppy/core';
describe('MyComponent', () => {
it('renders', () => {
jest.mock('@uppy/core', () => {
return {
Uppy: jest.fn().mockImplementation(() => {
return {};
}),
};
});
new Uppy();
expect(Uppy).toHaveBeenCalledTimes(1);
});
});
The test fails with the following error message:
Matcher error: received value must be a mock or spy function
Received has type: function
Received has value: [Function Uppy]
It appears that the default export from @uppy/core is the Uppy class, so my expectation is that the provided code would mock the class' constructor, however this doesn't seem to be the case.
Package versions:
I think the problem is that you're using jest.mock() at the wrong time. In other words, I think it's too late to call jest.mock() inside a test case because the package has already been imported by that time.
I'm not sure what the canonical way of doing this is, but here is how React does it:
jest.mock() is calledglobalSetup option; as you can see, this option is the in the config.base.js file:module.exports = {
globalSetup: require.resolve('./setupGlobal.js'),
}
when the test script is run, it will choose the config based on the provided argument(s)
an example of such config would be this file which, as you can see, extends config.base, which is where the globalSetup option has been set
/* ... */
const baseConfig = require('./config.base');
module.exports = Object.assign({}, baseConfig, { /* ... */ });
/* ... */
So, I'd say that making use of that globalSetup option should solve the problem.