How to mock variable responses based on the url when fetch(url) is called using the Jest testing framework?

Question

I have a module that looks like follows:

calculate-average.js

const fetch = require('node-fetch') const stats = require('stats-lite') const BASE_URL = 'https://www.example.com/api' const calculateAverage = async(numApiCalls) => { const importantData = [] for (let i = 0; i < numApiCalls; i++) { const url = `${BASE_URL}/${i}` // will make requests to https://www.example.com/api/0, https://www.example.com/api/1 and so on.... const res = await fetch(url) const jsonRes = await res.json() importantData.push(jsonRes.importantData) } return stats.mean(importantData) } module.exports = calculateAverage

I tried testing it along the following lines but I am clearly way off from the solution:

calculate-average.test.js

const calculateAverage = require('../calculate-average') jest.mock( 'node-fetch', () => { return jest.fn(() => {}) } ) test('Should calculate stats for liquidation open interest delatas', async() => { const stats = await calculateAverage(100) // Should make 100 API calls. console.log(stats) })

What I need to do is the following:

Be able to specify custom varied responses for each API call. For example, I should be able to specify that a call to https://www.example.com/api/0 returns { importantData: 0 }, a call to https://www.example.com/api/1 returns { importantData: 1 } and so on...
If a request is made to a url that I have not specified a response for, a default response is provided. For example if a response is made to https://www.example.com/api/101, then a default response of { importantData: 1000 } is sent.

I would preferably like to do this only using Jest without depending on modules like mock-fetch and jest-mock-fetch. However, if the solution without using is way too complex, then I would be happy to use them. Just don't want to create unnecessary dependencies if not required.

mgarcia · Accepted Answer · 2020-02-22 09:41:00Z

Sure you can! You can use mock function mockResolvedValueOnce method to return a result for a specific call and mockResolvedValue to return the default result.

jest.mock('node-fetch', () => { const generateResponse = (value) => { return { json: () => ({ importantData: value }) }; }; return jest .fn() .mockResolvedValue(generateResponse(1000)) // default response .mockResolvedValueOnce(generateResponse(0)) // response for first call .mockResolvedValueOnce(generateResponse(1)) // response for second call .mockResolvedValueOnce(generateResponse(2)); // response for third call });

Note that we are returning an object with the json property so that it returns the json data when you call res.json() in calculate-average.js.

If you want to return a specific response based on the url parameter, you will have to mock the desired behaviour in the returned mock function for node-fetch. The following example will mock the returned value so that for URLs where the counter is greater than 100 it will return 1000. Otherwise, it will return the same value present in the url:

jest.mock('node-fetch', () => { return jest.fn((url) => { // Get and parse the URL parameter. const value = parseInt(url.split('/').slice(-1)[0], 10); return Promise.resolve({ json: () => ({ importantData: value > 100 ? 1000 : value }) }); }); });

Thanks! This works great. Any idea how to return custom responses for specific urls since the above works for specifying response for successive fetch calls but doesn't allow for specifying custom responses based on the url parameter.
I have updated my answer to take into account how you could use the url parameter in the URL to return a custom response.

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