.nycrc

{
  "include": [
    "foo.js"
  ],
  "reporter": "text-lcov"
}

foo.js

module.exports = function (a, b) {
  return a + b;
}

package.json

{
  "name": "test-nyc",
  "version": "1.0.0",
  "main": "index.js",
  "license": "MIT",
  "dependencies": {
    "mocha": "4.0.1",
    "nyc": "11.3.0"
  },
  "scripts": {
    "test": "mocha spec.js",
    "test:coverage": "nyc npm t"
  }
}

spec.js

const foo = require('./foo.js');
const assert = require('assert');

it('Passes', () => {
  let actual = foo(1, 1);
  let expected = 2;

  assert.equal(actual, expected);
})