Testing: mocking node-fetch dependency that it is used in a class method - testing

Question

I have the following situation:
A.js
import fetch from 'node-fetch'
import httpClient from './myClient/httpClient'
export default class{
async init(){
const response = await fetch('some_url')
return httpClient.init(response.payload)
}
}
A_spec.js
import test from 'ava'
import sinon from 'sinon'
import fetch from 'node-fetch'
import httpClient from './myClient/httpClient'
import A from './src/A'
test('a async test', async (t) => {
const instance = new A()
const stubbedHttpInit = sinon.stub(httpClient, 'init')
sinon.stub(fetch).returns(Promise.resolve({payload: 'data'})) //this doesn't work
await instance.init()
t.true(stubbedHttpInit.init.calledWith('data'))
})
My idea it's check if the httpClient's init method has been called using the payload obtained in a fetch request.
My question is: How I can mock the fetch dependency for stub the returned value when i test the A's init method?

Answer 1

Finally I resolved this problem stubbing the fetch.Promise reference like this:
sinon.stub(fetch, 'Promise').returns(Promise.resolve(responseObject))
the explanation for this it's that node-fetch have a reference to the native Promise and when you call fetch(), this method returns a fetch.Promise. Check this out

Answer 2

You can stub fetch() as decribed in the manual
import sinon from 'sinon'
import * as fetchModule from 'node-fetch'
import { Response } from 'node-fetch'
// ...
const stub = sinon.stub(fetchModule, 'default')
stub.returns(new Promise((resolve) => resolve(new Response(undefined, { status: 401 }))))
// ...
stub.restore()
Note, that fetch() is the default export from node-fetch so you neeed to stub default.

Answer 3

sinon.stub(fetch) can't stub a function itself. Instead you need to stub the node-fetch dependency from inside ./src/A, perhaps using something like proxyquire:
import proxyquire from 'proxyquire`
const A = proxyquire('./src/A', {
'node-fetch': sinon.stub().returns(Promise.resolve({payload: 'data'}))
})

Answer 4

#mrtnlrsn's answer does not work for files generated from TypeScript in NodeJS, because default is a generated property, each module which imports such a dependency has its own, so stubbing one does not affect others.
However, NodeJS gives access to imported modules, so stubbing works this way:
const nodeFetchPath = require.resolve("node-fetch");
const nodeFetchModule = require.cache[nodeFetchPath];
assert(nodeFetchModule);
const stubNodeFetch = sinon.stub(nodeFetchModule, "exports");
Make sure you use the same node-fetch module as the tested module, e.g. using yarn dedupe. Or build your own nodeFetchPath.

Answer 5

In my case, I found it useful to preserve the node-fetch functionality since my mock responses were already being supplied via nock
To accomplish this, I proxyquire'd the dependency as described in the answer above but wrapped a require call in a spy:
import proxyquire from 'proxyquire'
import { spy } from 'sinon'
const fetchSpy = spy(require('node-fetch'))
const moduleA = proxyquire(
'./moduleA',
{ 'node-fetch': fetchSpy }
)
...
expect(fetchSpy.args).toBe(...)