How to do a Unit test of a unit where there is an interaction with an external service?

Question

Node.js

Good afternoon.
How to make a Unit test of a unit where there is an interaction with an external service?
Used by NodeJS Code
example:

async SynchShopProds (task) {
        return new Promise((resolve) => {
            try {
                fetch('example.url, {
                    method: 'POST',
                    headers: {
                        'Content-Type': 'application/json; charset=UTF-8',
                        'Authorization': `Bearer ${task.accessToken}`,
                        'User-Agent': UserAgent
                    },
                    body: JSON.stringify({ "searchTerm":null, "offerStatus":null, "categoryCode":null, "cityId":null, "start":task.start, "count":100 }),
                })
                    .then(res => res.json())
                    .then(json => {
                        console.log(json)
                        const r = json
                        if (r.errors) {
                            if (r.errors[0].type === 'InvalidTokenError') {
                                resolve({
                                    status: false,
                                    errorType: 'InvalidTokenError'
                                })
                            }
                            return
                        }
                        if (r.totalCount) {
                            if (r.offers != null) {
                                let insertProducts = ''
                                let countPlus = 0
                                r.offers.forEach(function callback(value, index, array) {
                                    let name = value.masterProduct.name.replace(/\'/g, "''")
                                    insertProducts += `INSERT INTO products ("kaspiSku", "kaspiLink", "name", "thumbnailUrl") VALUES ('${value.masterProduct.sku}', '${value.masterProduct.productUrl}', '${name}', '${value.masterProduct.primaryImage.small}') ON CONFLICT ("kaspiSku") DO NOTHING;`
                                    countPlus++
                                })
                                console.log('COUNT PLUS', countPlus)
                                poolDB.connect((err, client, done) => {
                                    const shouldAbort = err => {
                                        if (err) {
                                            console.error('Error in transaction', err.stack)
                                            client.query('ROLLBACK', err => {
                                                if (err) {
                                                    console.error('Error rolling back client', err.stack)
                                                }
                                                done()
                                            })
                                            resolve({
                                                status: false,
                                                errorType: 'DatabaseError'
                                            })
                                        }
                                        return !!err
                                    }
                                    client.query('BEGIN', err => {
                                        if (shouldAbort(err)) return
                                        client.query(insertProducts, (err, result) => {
                                            if (shouldAbort(err)) return
                                            client.query('UPDATE "synchProductsTasks" SET "totalCount" = $1, start = start + $2 WHERE "shopId" = $3', [r.totalCount, countPlus, task.shopId], (err, result) => {
                                                if (shouldAbort(err)) return
                                                client.query('COMMIT', err => {
                                                    done()
                                                    if (err) {
                                                        console.error('Error committing transaction', err.stack)
                                                        resolve({
                                                            status: false,
                                                            errorType: 'DatabaseError'
                                                        })
                                                    } else {
                                                        resolve({
                                                            status: true,
                                                            stage: 'next',
                                                            errorType: null
                                                        })
                                                    }
                                                })
                                            })
                                        })
                                    })
                                })
                                return
                            } else {
                                resolve({
                                    status: true,
                                    stage: 'finish',
                                    errorType: null
                                })
                            }
                            resolve({
                                status: true,
                            })
                        }
                    })
                    .catch(err => {
                        resolve({
                            status: false,
                            errorType: 'RequestError'
                        })
                    })
            } catch (err) {
                console.error(2, err)
                resolve({
                    status: false,
                    errorType: 'ServerError'
                })
            }
        })
    }

Answer 1

You need to refactor this module so that it does not make a request to an external service directly.
Then it will be possible for him, as part of the test, to slip a stub instead of a real client.