Testing the Redux saga API call using Jest with an image response

Having trouble testing a fetch that returns a readable stream for an image. Not sure how to approach this specific case. Is there a simple way to mock the call's response?

Any assistance would be greatly appreciated :D

This is how the generator function looks like:

export function* getImageSaga () {

    try {
        const headers = {
            Accept: 'image/jpeg',
        };
        const options = {
            headers,
            method: 'GET',
        };

        const response = yield call(fetch, GET_IMAGE_API, options);
        const blob = yield response.blob();
        const url = URL.createObjectURL(blob);
        yield put(getImageSuccess(url));
    } catch (error) {
        yield put(getImageError(error));
    }
}

Answer №1

If you want to test the saga generator function step by step, consider following this approach: testing the saga generator function.

Before testing the saga generator function, there are a few things to keep in mind:

  1. jsdom currently does not support fetch, as highlighted in issue#1724

  2. jsdom also lacks support for the URL.createObjectURL API, as mentioned in issue#1721

  3. jestjs uses jsdom as the default test environment for simulating a browser-like environment.

To address these limitations, use the isomorphic-fetch polyfill for fetch, mock the URL.createObjectURL() API and its return value. Additionally, consider leveraging the Response() constructor to create custom Response objects:

const response = new Response();

This custom response object serves as the mock return value of fetch.

The testing scenario involves the following files:

index.ts:

import { call, put } from 'redux-saga/effects';

// Additional code here...

index.test.ts:

import { call, put } from 'redux-saga/effects';
import { getImageSaga } from './';

// Additional code here...

After running the tests, the results indicate that the image retrieval was successful, with relevant coverage statistics displayed.

Further configuration settings can be found in jest.setup.js and jest.config.js files included in the project.

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