How to filter out certain patterns from mocha/nyc test coverage (Node.js)?

In mocha.opts, I am sure how to set-up coverage pattern that needs to be taken into account but not sure how to include the patterns that has to be filtered-out?

For example,

I have the file mocha.opts containing coverage patterns which is passed as param to the following command:
nyc mocha --opts ./mocha.opts

The content of the mocha.opts is given below:
test/tests/routes/*.test.js

But there are lot of custom js scripts imported into *.test.js files. But these custom js scripts contains functions which I don't want to be covered under coverage report nor I have written unit test for them.
Is there any way to filter out these patterns from coverage by declaring them into mocha.opts file?

Solution

In package.json, you can add nyc configuration. Something like this. https://www.npmjs.com/package/nyc#excluding-files

"nyc": {
  "include": [
    "./**/*.js"
  ],
  "exclude": [
    "./test/",
    "./db/migrations/"
  ]
}

I haven't seen a way to specify in mocha.opts though.