如何使用chai在redux-sagas生成器功能测试用例中测试catch块？

Question

export function* testGeneratorFunction() {
   try {
       yield put(showSuccess(success));
   } catch (error) {
       yield put(showError(error));
   }
}

In the above function i need to test the catch block. I have to test whether showError() function is called or not.

Answer 1

You might want to use an helper library for that, such as redux-saga-testing .

Disclaimer: I wrote this library to solve that exact same problem

For your specific example, using Jest (but works the same for Mocha), you would write:

import sagaHelper from 'redux-saga-testing';
import { call, put } from 'redux-saga/effects';
import { showSuccess, showError } from './my-actions';
import { api } from './my-api';

function* testGeneratorFunction() {
    try {
        const data = yield call(api);
        yield put(showSuccess(data));
    } catch (e) {
        yield put(showError(e.message));
    }   
}

describe('When testing a Saga that throws an error', () => {
    const it = sagaHelper(testGeneratorFunction());

    it('should have called the API first, which will throw an exception', result => {
        expect(result).toEqual(call(api));
        return new Error('Something went wrong');
    });

    it('and then trigger an error action with the error message', result => {
        expect(result).toEqual(put(showError('Something went wrong')));
    });
});

describe('When testing a Saga and it works fine', () => {
    const it = sagaHelper(testGeneratorFunction());

    it('should have called the API first, which will return some data', result => {
        expect(result).toEqual(call(api));
        return 'some data';
    });

    it('and then call the success action with the data returned by the API', result => {
        expect(result).toEqual(put(showSuccess('some data')));
    });
});

I modified your example slightly, because "put" would never throw an exception.

In real life, a call to an API or some other logic would be more likely to throw an exception.

You'll find plenty of other examples (more complex ones) on the project's GitHub.

Answer 2

There is a decent chapter on Error Handling in the manual. In short, you can use .throw instead of .next to trigger the catch block in your saga:

assert.deepEqual(
  gen.throw({message: "error!").value, // <-- notice the throw instead of next
  put({ type: 'PRODUCTS_REQUEST_FAILED', error }),
  "fetchProducts should yield an Effect put({ type: 'PRODUCTS_REQUEST_FAILED', error })"
)

I'm not familiar with Chai syntax, but for instance using Jest's expect it would be:

expect(gen.throw({message: "server ded"}).value).toEqual(
  put({ type: 'PRODUCTS_REQUEST_FAILED', error })
)