How to call a promisified function in-page with Nightmare/mocha/chai

Two important things to keep in mind:

1. You have to return from inside the "it" function (boldfaced below)
2. You need to list parameters to evaluate three times, the third time as a list of parameters after the function argument to evaluate (also boldfaced below)

    it(`Keystretching ${clearTextPassword} with ${hexNonce} with current params should give ${theExpectedDerivedKey}`, function () {
      this.timeout('10s')
      return Nm()
        .goto(`file://${__dirname}/../index.html`)
        .evaluate((clearTextPassword, hexNonce, keyStretchFactor) => {
          return stretch(clearTextPassword, hexNonce, keyStretchFactor)
        }, clearTextPassword, hexNonce, keyStretchFactor)
        .end()
        .then(res => expect(res.derivedKey).to.equal(theExpectedDerivedKey))
    });