Unit test a private method that uses request, pipe and stream using mocks

Question

I want to unit test the exported method in the code below. I want to mock the values in the private method to control the reject/resolves of the returned Promise. client is node-postgres object that is already connected to the database.

I know I can use proxyquire to stub out required libraries but how can I mock the chained methods .on('error', ...), .pipe(stream) and .on('end', ...) such that I can control the returned values.

Note the exported method shown is a simplification of the real one and exporting importDomain is not feasible.

const copyFrom = require('pg-copy-streams').from
const request = require('request')
const Promise = require('bluebird')

// private
function importDomain (client, domain) {
    return new Promise((resolve, reject) => {
    let stream = client.query(copyFrom(`COPY ${domain.table} FROM STDIN;`))

    let req = request(`${domain.url}`)
    req.on('error', reject)
    req.pipe(stream)
       .on('error', reject)
       .on('end', resolve)
  })
}

// public
module.exports = (client) => {
  let domain = someFunctionReturningDomain()
  importDomain(client, domain)
}

Answer 1

In order to unit test the function importDomain, you will need to mock request. It's required by your module so should be considered to be tested and working normally.

Since you provide the client as a parameter. Its query method and the stream returned is already under full control out of the module.

Simply by replacing req.pipe, you will be able to control what is passed to the stream your client created.