in my app.js I have this function:

const bodyParser = require('body-parser')

const express = require('express')

const app = express()

const cors = require('cors')

app.use(bodyParser.urlencoded({ extended: true }))

app.use(bodyParser.json())

const dataFilePath = '../shoppinglist/data.json'

const baseUrl = '/api/v1/shoppingLists'

let client

const fileSystem = require('fs')

let data







app.post(baseUrl, (req, res) => {

    const newData = req.body

    if(newData != null && !isEmpty(newData) && isRequiredFieldGiven(newData)){

        readUpdatedData()

        newData.id = getNewestId()

        data.data.push(newData)

        //updateData(data)

        console.log(data)

        res.setHeader('Content-Type', 'application/json')

        res.statusCode = 201

        res.json({"Location" : baseUrl + '/' + newData.id})

    }else{

        res.statusCode = 400

        res.send()

    }

})



function readUpdatedData(){

    let rawData = fileSystem.readFileSync(dataFilePath)

    data = JSON.parse(rawData)

}

in the testfile, I'm doing this to test an API post call which is using the readUpdatedData in it:

const request = require('supertest')

const shoppingListDataPath = ('../shoppinglist/data.json')

const baseUrl = '/api/v1/shoppingLists'

const appPath = '../src/app'



describe('create list entry', () => {

it('should return 201', async () => {

   let fs = require('fs')

 jest.spyOn(fs, 'readFileSync').mockReturnValueOnce(JSON.stringify({"some" : "data"}))



    let {app} = require(appPath)

        const entry = {

            "id": 1,

            "name": "filled shopping list2",

            "location": "lidl2",

            "targetDate": "22.03.1986",

            "priority": 1,

            "isFinished": false,

            "items": [{"count": 1, "name": "vodka"}, {"count": 1, "name": "vodka"}

            ]

        }

        const res = await request(app)

            .post(baseUrl)

            .send(entry)

        expect(res.statusCode).toEqual(201)

        expect(res.body).toEqual({"Location": baseUrl + 1})

    })

})

When I'm using mockImplementationOnce or mockReturnValueOnce, I've get an error 500 while it's calling readFileSynch.
I've changed it to mockImplementation or/and mockReturnValue, then i've got an error 400. It doesn't call then the app.post all.

I've also tried with jest.fn() but was the same result.