Mocha-webpack not found 'fs' module

I'm working mocha and chai unit test. It's working fine, but if I add mongoose to test file as below:

let mongoose = require('mongoose')

I'm getting error.

https://firebasestorage.googleapis.com/v0/b/testproject-49172.appspot.com/o/Capture.PNG?alt=media&token=268902db-2b39-4c11-a299-c9666480e873

Is anyone can help me? Thanks.